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0)u6K»9er« of ** t^ dif anb eofourman^* ^oumaf ' 






In the following pages I have collected the more 

important facts connected with the manufacture of 

Glue and allied products together with the experience 

I have gained in examining various commercial 

samples during the past ten years which have been 

used for various purposes. In this work I have 

been ably assisted by Mr. C. G. Stewart, who has 

also rendered me considerable help in revising these 
notes for the press. 


28 Victoria Street, S.W., 
February, 1900. 




Constitution and Properties . , 1 

Definitions — Sources — Gelatine — Chondrin and allied bodies 
— Physical and chemical properties — Classification — 
Grades and commercial varieties. 


Raw Materials and Manufacture 21 

Glue Stock — Liming — Extraction — Washing and clarifying — 
Filter presses — Water supply — Use of alkalies — Action of 
bacteria and of antiseptics — Various processes — 
Cleansing — Forming — Drying —Crushing, etc. — Secondary 


Uses of Glue . 65 

Selection and preparation for use — Carpentry — Veneering — 
Paper making — Bookbinding — Printing rollers — Hecto- 
graphs — Match manufacture — Sandpaper, etc. — Substi- 
tutes for other materials — Artificial leather and 


Gelatine 79 

General characters — Liquid gelatine — Photographic uses — 
Size — Tanno- chrome and formo- gelatine — Artificial Silk 
— Cements — Pneumatic tyres — Culinary — Meat extracts 
— Isinglass — Medicinal and other uses — Bacteriology. 




Glue Testing • ... 107 

Review of processes — Chemical examination — Adulteration 
— Physical tests — Valuation of raw materials. 

Commercial Aspects 136 



1. Filter Press 31 

2. Hollander 38 

3. Jacketed Kettle 42 

4. Boiler for Glue Extraction .44 

5. Vacuum Pan 48 

6. Yaryan Evaporator 49 

7. Orosseill Bone Mill 50 

8. Orosseill Bone Mill 51 

9. Hewitt's Machine 61 

10. Leuner's Apparatus 63 

11. Steam Glue Ovei^ 68 

12. Kissling's Consistency Apparatus 120 

13. Enqler's Viscosimeter 128 

14. Rideal-Slotte Viscosimeter 130 



Like many commercial terms, the words *'glue" and 
''gelatine/' as indicating distinct substances, are not 
strictly defined. Gelatine, from the Latin gelare, to con- 
geal, is held to imply, commercially, "animal jelly; glutin- 
ous material obtained from animal tissues by prolonged 
boiling " ; this of course would include glue, and accord- 
ingly glue (Fr. glu, later Latin glws, "akin to gluten, 
from gluere, to draw together". — ^Webster) is described as 
" a hard, brittle, brownish gelatine, obtained by boiling to 
a jelly the skins, hoofs, etc., of aninials. . . . The name 
is also given to other adhesive or viscous substances," 
like "marine glue," which contains caoutchouc and resins, 
but no gelatine. 

In the ordinary sense, the words are defined with 
sufficient sharpness by the appearance and the physical 
characters on which are based the uses of the two^ pro- 
ducts. Gelatine is made in pale or nearly colourless thin 
sheets, and is used for purposes in which absence of 
taste, odour and colour, with firmness of jelly and easy 
solubility, are required. Glue, on the other hand, is em- 
ployed for its adhesiveness, stiffness and elasticity, and 
met with in the familiar thick and darker sheets. 

To the chemistry of gelatine more attention has been 
paid than to that of glue, as the former is a purer and 
more definite substance, and when prepared from different 
sources, exhibits considerable uniformity in composition 


and properties. Neither glue nor gelatine exist ready 
formed in nature : they are products of the action of iieat 
and water on many nitrogenous animal tissues related to 
the albuminoids proper, but not identical with them^ 
Allen ^ proposes to avoid the term "albuminoid** wherever 
possible, and to distinguish the two classes as proteida, 
including fibrin, albumin, casein, etc., and proteoida, as 
gelatin and its congeners.^ The latter are insoluble in 
cold water, and in weak acids and alkalies, but some of 
them swell up in cold, and dissolve in boiling water, 
the latter more easily when the operation is conducted 
in a closed vessel, so that the Hquid is "superheated,** 
or its boiling point raised' by pressure. This group or 
subclass are called collagens, or gelatoids, because their 
solutions become more or less gelatinous on cooling. 
They include the following bodies. 

Collagen or Ossein, the chief component of the white 
fibres of . connective tissue from the skin, tendons, muscles, 
bones, intestines, and the swimming bladders of fish. 
It may be prepared by exhausting minced tendons with 
water, soaking in lime water to dissolve mucin, then 
washing with water, dilute acetic acid, and again with 
water, finally gently drying, and exhausting with ether 
to remove any fat. The product may still contain nuclein 
"and elastin. 

Trypsin, a ferment from the pancreas, has the power of 
digesting ** all the tissue elements except the true collagenous 

1 Com/mercial Organic AnalysiSy 1898, iv., p. 456. 

^TMs distinction is convenient, as indicating many group-difEerences, 
but cannot imply any fundamental difference in chemical constitution. 
Maly {Monats, Chem,^ x., 26), from experiments on the oxidation of aJbumin 
and gelatin, considers that '* gelatin is as much a proteid as fibrin or casein, 
and that a classification into albuminoids and proteids cannot be upheld ". 
Buchner and Curtis, by a different treatment (Beric/i^, xix., 850), obtained 
similar decomposition products from gelatin and albumin, and speak of this 
as a fresh proof of their analogy in chemical properties. 


fibrils/' so that it may be used to prepare a very pure 
collagen. This might in turn be useful as a source of 
specially pure gelatine. 

Bones in the dry state contain about 32 to 33 per cent, 
of organic matters, consisting mainly of oaaeiji — believed to 
be identical with collagen from tendon and skin — with small 
quantities of elastin (an insoluble nitrogenous body from the 
membranes lining the Haversian canals, lacunae, and canali- 
culi) proteids and nuclein from the bone corpuscles, blood, 
marrow, nerve-fibres, etc., and a small quantity of fat, even 
after removal of the marrow. The average composition of 
undried bone has been given as : — 

Water 61-0 

Fat 16-7 

Ossein, etc. 11*4 

Mineral matters 21*9 


But the percentage of water and fat in this, as in other 
forms of glue stock, is very variable ; therefore, in buying in 
large quantities, it should be always previously determinedi 

The inorganic constituents of bone are : calcium phos- 
phate, about 60 per cent. ; calcium carbonate, 6 to 7 per 
cent. ; magnesium phosphate, calcium fluoride, and sodium 
chloride, 1 to 2 per cent, of the dry substance. These are 
contained in the. bone ashy of which we shall speak later. 

When a bone is treated with ether and petroleum to 
remove fat, and then soaked for some time in dilute hydro- 
chloric acid, the mineral salts dissolve, and a skeleton of the 
animal matter is left of the original shape of the bone, but 
soft, elastic and semi-transparent. It is better to use acid of 
not more than 5 per cent., and to renew it occasionally, as a 
stronger acid and long contact is apt to rot the ossein, and 
is difficult to wash out. The phosphates can be precipitated 
from the solution by ammonia. After thorough washing, 


the ossein is neutral, tasteless, and contains little ash ; on 
drying, it becomes hard and somewhat brittle. Boiling 
with water of course converts it into gelatin, more rapidly 
in the presence of acid, but in this case gelatin-peptones, 
or ** gelatones,** which do not gelatinise, are liable to be 

It was formerly stated that collagen and gelatin had the 
same elementary composition, and that the change in pro- 
perties was merely a physical one, similar to many cases of 
physical isomerism that are known in chemistry. 

This idea is supported by the fact that the analyses of 
purified bone-cartilage, isinglass, and gelatine from connec- 
tive tissue, after drying, give approximating results. They 
are calculated as free from ash. 










Carbon . 




49-38 . 














Oxygen . 




J 0-71 ( 
« 26-13 ♦ 







But the opinion now held is that the change is really a 
chemical one of hydrolysis, or combination with water, by 
which collagen is first converted into gelatin, then into 
gelatosea (analogous to the proteoses or albumoses derived 
from true proteids), next into gelatin-peptones or gelatones 
(similarly analogous to peptones from true proteids), finally 
into amido-acids, lysine, lysatine, etc. 

Hofmeister considers gelatin as a definite hydrate of 
collagen, which is re-converted into the latter by heating to 
130° C. This would explain the correspondence between the 


analyses of the dried substances. He suggests the following 
formulsB and equations : — 

Ck>llagen. Gelatin. 

O102H149N31O38 + H2O = Oio2HigiNsi039. 

Gelatin. Semi-glutin. HemicoUin. 

OioaHwiNgiOa, + 2HaO = C«H«N„Ojb + C^^H^oNi^Oie. 

The two latter bodies being gelatones, the first sparingly, the 
latter easily, soluble in 70 to 80 per cent, alcohol. He passes 
over the formation of gelatose.^ 

Gelatin.^ — When pure, a hard glassy substance possessed 
of considerable elasticity, neutral and free from odour, taste, 
or omc il, swelling up but not dissolving in cold water, insol- 
uble in strong alcohol, ether, hydrocarbons or oils. It is a 
typical member of the class of colloids {xoWa, glue), charac- 
terised by their gelatinous nature, non-diflfusibility, and absence 
of crystallisation. Gelatin contains a small proportion of 
sulphur, which it has been argued is due to 9in impurity, as 
it varies in different samples, and is stated to be sometimes 
absent. Thus A. H. Allen quotes Schlieper as having found 
0*12 to 0'14 per cent, in the products from bone and ivory, 
Von Bibra ** always very appreciable quantities in bone 
gelatin," Hammarsten 0*7 per cent, in fine commercial 
samples yielding 1*74 per cent, of ash, while in his own 

^The above formulae are only einpiricalf and give no clue to the chemical 
structure. Schiitzenberger and Bourgeois propose C^a^iu^ifiw ^^^ *^® 
formula of gelatin : the former, from a study of the action of barium 
hydroxide, concludes that ossein is built up of three groups : — 

1. A nitrogenous nucleus of urea or oxamide. 

2. A series of acids CnH2nN20g. 

8. Fatty amido-acids GnH2n + i^Og, starting from amido-acetic acid or 
glycocine. It is certain that the latter group continually figure in its decgm- 

^ It would be convenient that, when the pure substance is spoken of, the 
word should be spelt gelatin^ as it is in most chemical treatises ; when the 
commercial article is meant the ordinary spelling gelatine should be retained. 
The word glutin is sometimes used, especially on the Continent, but is objec- 
tionable on account of the risk of confounding it with gluten, from cereals. 


laboratory he found 0*17 per cent, in Nelson's, and 0*38 per 
cent, in Brazilian isinglass. 

When heated, gelatin softens, swells and gradually darkens 
with an odour of burnt feathers, emitting ^Ikali n^fumes con- 
taining amylamine, CgHjiNHg, and other bodies, then burns 
with a sooty flame leaving a hard charcoal which slowly bums 
away to a white ash amounting to about 1 J per cent. 

The changes effected by bacteria in gelatin and glue will 
be discussed in Chapter IV. 

The most characteristic property both of gelatin and 
glue is the action of water. Gelatin and the higher 
grades of glue swell up in cold water without dissolving, 
absorbing up to ten times their weight, and forming a 
soft mass which liquefies completely on warming. In hot 
water all forms of gelatin dissolve; the solution, if weak, 
remains liquid on cooling, but, if stronger than 1 per cent., 
it sets to a jelly. The limit of gelatinisation perceptible 
at 0° C. is about 1 in 150. A jelly of 6 per cent, strength 
is firm even in summer. Particulars of water absorption, 
firmness of jelly, etc., will be given in Chapter V., on Testing. 
The solution rotates polarised light strongly to the left : the 
degree of rotation approximately ascertaining the strength. 


If the swollen gelatin mentioned above be subjected 
for twenty-four hours to a stream of cold water, then 
washed four or five times with distilled water, the salts 
and many impurities diffuse out. The jelly is liquefied by 
heat, adding boiling water if necessary, filtered by pressure 
through a fine filter in a hot-water-jacketed funnel, and 
poured slowly, with agitation, into a large quantity of 90 
per cent, alcohol. The gelatin is almost entirely pre- 
cipitated in flocks which agglutinate and settle, the alcohol 
containing some gelatin, and extractive matters is de- 


canted and recovered by distillation. The solution and 
precipitation is twice repeated, and the product washed 
with absolute alcohol and ether, dried in vacuo, finally 
by a heat not exceeding 40° C. It is then white, friable, 
and contains very little ash or sulphur. 

Davidowsky prepares **pure glutin " from buckshom 
by extracting the salts with hydrochloric acid, washing, 
removing the fat by lime water, boiling, and precipitating 
the liquefied jelly by alcohol. He states that ** the coher- 
ence varies with the source". . 

Most varieties of tannin give with solutions of gelatin 
an almost insoluble curdy precipitate, which dries up to a 
hard, elastic, imputrescible mass. This reaction is con- 
nected with the production of leather; its chemical rela- 
tions will be discussed in Chapter V. The other properties 
of gelatin will also be alluded to in subsequent sections. 

Allen gives the following table of gelatin-producing 
substances and their congeners, whicii is valuable as 
showing what substances will not produce glue, as opposed 
to numerous patents which claim miscellaneous raw pro- 
ducts as available for the manufacture: — 

A. GoirLAGENS or Gelatoids. 

Dissolved more or less easily by boiling water, the 
solutions gelatinising on cooling. TQontain only a small 
quantity of sulphur,. 

1. Collagen or ossein, from bones, skin, etc. 

2. Chondrigen, from permanent cartilages. 

3. Isinglojss^ from fish bladders. 

4. Sericin, from silk. On boiling silk with water 
under pressure it yields a yellowish transparent substance 
which gives a strong jelly, and resembles gelatin in being 
precipitated by alcohol or tannin, but is distinguished from 


it by giving a precipitate by potassium ferro-cyanide. But 
waste silk is usually utilisable for other purposes. 


Elastin, from the elastic fibres left after the removal, 
by boiling, acids and alkalies, of the more soluble com- 
pounds of certain ligaments, especially the neck-band of 
the ox, ligarrhentum nuchce. It often forms a residue of 
glue manufacture, removed in the settling, as it does not 
yield gelatin, nor dissolve without decomposition. 

Fibroin is a similar insoluble substance obtained from 
silk and from spiders' webs. 

C. Chitinoids. 

Ohitin, from the wing-cases of beetles, or the shells 
of crabs and lobsters, can be converted by strong acids 
into a gelatinous form, which, however, is of no utility. 

Conckiolin, from shells of moUusca, is similar in pro- 
perties, but differs in composition. 

Spongin, from sponges, on boiling with water yields 
no gelatine. 

D. Keratoids. 

Keratin, from horns, hoofs, feathers, hair and wool, 

will be further alluded to in Chapter II. 


Certain materials, such as young bones while still in the 
soft state, and the ** permanent *' cartilages, such as those 
of the ribs and joints, consist mainly of a substance called 
**chondrinogen" or ** chondrigen," related to ossein, and 
yielding, on boiling, a body named "chondrin," which re- 
sembles gelatine, but is weaker in gelatinising and adhesive 
properties, and is distinguished by a few differences in 
chemical behaviour. 

It is prepared pure by boiling with water and precipitat- 
ing with alcohol in a similar manner to gelatin, and then 
manifests like properties, except that its jelly has less 


strength. Some recent researches tend to show that chon- 
drin is a mixture of gelatin and mucin, but commercially the 
presence of the substance called chondrin, which is present 
in many glues, may be held to be detrimental ; it is useful, 
however, for size. It is distinguished by giving, like mucin, 
a precipitate with acetic acid. For photographic work it is 
specially objected to, and may be tested for by adding a 
saturated solution of chrome alum to a 10 per cent, solution 
of the suspected gelatine in hot water, and well stirring, 
"when, if chondrin be contained in injurious quantity, the 
liquid will gelatinise while still hot". But, unless a pre- 
cipitate is also given by acetic acid, this test cannot be 
considered conclusive. Most photographic gelatines contain 
but little chondrin. 

This name includes a number of slimy bodies, present in 
many tissues, which swell up in water but do not really 
dissolve. They are soluble in lime water and soda, there- 
fore are ordinarily removed with the Uquors of liming. Salt 
not stronger than 10 per cent, dissolves them, but when 
more concentrated they are precipitated. They have no 
adhesive nor gelatinising power, and show a strong tendency 
towards ** foaming," therefore their presence should be 
avoided in glue manufacture. The use of alkalies in some 
processes of treating is found to encourage foam in the glue 
produced, a fault that is also found more commonly in glues 
made from salted than from limed hides, two facts that seem 
to point to the hitherto unobserved presence of mucin. The 
addition of acetic acid will reveal the impurity by causing a 
precipitate in the liquor.^ The mucins are also thrown down 
by alum, so would be removed with other things in this 
method of clarifying. 

* The addition of a small quantity of acetic acid to the glue liquor might 
be tried as a remedy for foaming, afterwards straining or settling, and 
neutralising with soda, not chalk. 


The statements are continually recurring that ** gelatine 
is the purest form of glue,*' and that " glue is impure gela- 
tine'*. There is no doubt that this is only partially true. 
Just as gelatin is a product of the action of heat and water 
on collagen,[so glue is the result of the same action on 
gelatin, the hydrolysis being carried further. Brannt, 
following Davidowsky, holds that **glue is the product of 
several transformations : — 

"I. In drying the hide. Because the result of boiling 
green hide, prepared in the usual manner by liming, etc., 
but not dried, will be an entirely different product of less 
consistency^ than that .obtained by drying the hide after 
liming and then boiling. 

** II. In boiling, 

"III. In drying the obtained jelly. This nrtiy explain 
the fact that the jelly, which is not yet converted into glue, 
differs in its behaviour from glue solution. 

** IV. It is well known that when boiled for some time 
glue does not gelatinise. We distinguish therefore : — 

** (a) Glue-yielding substance. 

** (b) Crude glue. 

**(c) Jelly. 

"(d) Glue. 

** We believe that, by reason of our experience, we are 
justified I inj making this distinction between glue-yielding 
substance and crude glue. Lime carefully,, fresh calves' 
heads,lsuch[as the tanner cuts off after raising the skin, and 
then boil them without previous drying — the result will be a 
turbid soup,^containing, though everything be dissolved, no 
jelly whatever, or at least very httle. 

** (6) Crude glue is glue-yielding substance freed from 
foreign matter, and physically prepared by drying, 

" (c) Jelly, Its 'adhesive power is far less than that of 
solution of finished glue, and it will become more quickly 



putrid than the latter/' because (1) it contains more water ; 
(2) it is not so completely sterihsed ; (3) it does not contain 
the empyreumatic substances of glue which are antagonistic 
to bacteria. ** At 21° C. a * jelly' putrifies under twenty- 
four hours, while glue solution can be kept much longer." 

To render clearer the practical differences between glue 
and gelatin, I instituted the following experiments : — 

A 5 per cent, and a 50 per cent. ^ solution of (a) 
'' Coignet's gold medal " gelatine in thin transparent sheets, 
and (6) a dark Scotch glue of average quality, were prepared 
by heating carefully with water to 70° C. in wide-mouthed 
stoppered bottles, and stirring so as to dissolve as soon as 
possible. The tenacity of the stronger solutions was deter- 
mined between stone-china blocks as described in Chapter V., 
while the consistency of the jelly at 0° C. and the viscosity 
of the hquid at 25° C. were ascertained in the 5 per cent, 
solutions. The results were tabulated : — 


h is. 

OQ p. 






weight: lb. 

per sq. inch. 

ency of 
jelly at 


at 25<» C, 









Glue . 

Glue . 

neutral, im- 

Strong odour, 
very viscid, 
slightly al- 

Like the 60 
per cent. 

Like the 60 
per cent. 

Pale, tough, 

Dark browu, 
hard, rather 

Nearly clear, 
stifi jelly. 

Liquid at 
ture, a 
weak jelly 
at 0° C. 



• • • 

• • • 


• • • 

• • • 

■ • • 
• ■ • 


• • • 

• • • 


^ There is often some confusion in stating the strength of glue solutions^ 
three distinct methods of making up a nominally 6 per cent, solution being 
used : (1) 6 grammes glue pltis 100 cc. water ; (2) 6 grammes in 100 cc. of the 


The "units** of consistency require some explanation. 
The 5 per cent, liquids (a) and (b) being reduced to 0° C. 
by a bath of ice and water, a glass tube 13 millimetres 
diameter was supported vertically, so as to move easily 
between guides, its lower closed and rounded end resting on 
the jelly. It was then weighted with shot (in later experi- 
ments with mercury) till it began to descend steadily, and the 
weight of shot plus tube ascertained. The figures were : — 

(a) 232*5 grammes sank 50 millimetres in 270 seconds. 
(6) 20-3 „ „ ., „ 300 „ 

^^j 232-5 X 270 ^ ^g gg^ ^^.^^ 
6 centimetres 

(b) ^-3 ^ ^ = 1,218 „ 


(See Chapter V., p. 122.) 

As the 60 per cent, gelatine, when hot, was of such a 
consistence that it was nearly impossible to work with, 
continually forming skins, and giving too thick a layer 
between the blocks, a weaker solution of each substance 
was tried for tenacity. 

Breaking strain of 33 per cent, solutions of glue and 
gelatine, mounted between blocks of old mahogany, area 
of surfaces 1 square inch, compressed by a weight of 4 lb., 
for forty-eight hours at 23° C. in a dry room. 

Gelatine 6601b. 

Glue 6101b. 

To ascertain the action of heat on the solutions : — 
Experiment 1. With concentration, 100 cc. of the 
5 per cent, liquids were evaporated down in a water bath 
at 96° C. till 10 cc. was left: the solutions ' were now of 
60 per cent, strength. The formation of a skin was 
avoided by occasional agitation. The gelatine, owing to 

made solution ; (3) 5 grammes in 100 grammes of solution. The method of 
preparation being often omitted, the results of published experiments are not 
strictly comparable. In these experiments method (2) has been adopted as 
more convenient. Evaporation in all cases must be avoided. 



its viscidity, required a much longer time to evaporate — 
twelve hours, as against five for the glue. Cooled to room 
temperature (22° C), the gelatine set, but was much less 
firm than the original 50 per cent, solution, while the 
glue was simply a slimy liquid. The tenacity was again 
tried as above. The results were : gelatine 602, glue 
184 lb. Therefore, the heating had slightly improved the 
tenacity of the gelatine, but reduced that of the glue. 

Experiment 2. Without concentration. Two quanti- 
ties of 300 cubic centimetres of glue and gelatine were 
heated in closed bottles for seventy-two hours continuously 
to 100° C, then left for forty-eight hours at room tem- 
perature. Both were now quite liquid, and remained so 
even at 0° C, therefore the gelatinising property had 
entirely disappeared. In the glue the smell had become 
intensified; the gelatine remained clear, but had acquired 
a yellow-brown colour and a gluey odour. The viscosities 
were : gelatine, 13 ; glue, 13*5. 

A chemical examination was also made of the two liquids. 
As the bottles had been carefully sealed, the total nitrogen 
would be the same as before the heating. Distilled with alka- 
line permanganate, they gave the same amount of ammonia 
before and after heating. 

To ascertain whether the liquefaction affected by heat was 
of the same character as that caused by enzymes, whether 
from digestion or bacteria, the solution of gelatine was treated 
with ammonium sulphate, which throws down gelatin and 
gelatose, but not peptones or gelatones. The precipitate was 
nearly the same in amount as that obtained from fresh 
commercial gelatines, whereas after artificial digestion or 
bacteriolysis the precipitate would be much less. The con- 
clusions drawn from these researches are : — 

1. That gelatine and glue are distinct substances, the 
latter formed from the former by heat and evaporation. 


2. That the action of heat is different from the peptonisa- 
tion caused by digestion or bacteria. 

3. That gelatine in turning into glue loses gelatinising 
power, becomes more liquid, and somewhat increases in 

4. That after about twelve hours' heating all the promi- 
nent physical properties of value decline, while the chemical 
reactions are little altered. 

Hence it might be possible to restore the qualities of a 
glue which had been spoilt by heating. With this view the 
effect of formaldehyde, which in small quantity is known to 
increase the stiffness of jelly, was tried. The liquid resulting 
from the seventy-two hours' heating gave, on evaporation, a 
hard, transparent, brittle residue, easily soluble in ice-cold 
water, and almost destitute of adhesive power, therefore not 
gelatine. Addition of 1 in 2000 formaldehyde did not precipi- 
tate the liquid, but after the evaporation left an opaque brittle 
residue, almost insoluble, and not adhesive. No restoration 
seemed to have been effected in this way. 

The ** souring ** of glue liquors is due to bacterial action. 
Some notes on the point will be found in Chapter IV. 

Classification of Glues. 

The chief commercial varieties of gelatine or glue can be 
roughly divided into two groups, subdivided again by the 
source from which they are derived : — 

I. Olues proper. — ^Varieties chiefly used for their adhesive- 
ness and tenacity : — 

(a) Skin, hide, or leather glue. 

(6) Bone glue. 

(c) Fish glue. 

II. Oelatines, — Those used principally for their gelatin- 
ising properties, and differing from ordinary glue in their 
purity and freedom from colour and odour. They are also 


used for filling, clarifying, glazing and stiffening, and for all 
purposes where whiteness, transparency, and absence of 
colour, odour, and taste are demanded. They are all obtained, 
like glues, by the action of heated water, sometimes assisted 
by dilute acids, on materials of special selection. 

{d) Skin gelatine, from carefully cleaned and treated 
intercellular connective tissue, such as skin, tendons, ears, 
intestines, or even the sheaths of mu'scular fibre. 

(e) Bone gelatine. Crushed bones yield about one-third 
of their weight of an excellent gelatine if exposed on trays or 
gauze, in a closed steam - jacketed vessel, to a regulated 
descending flow of water, and an ascending current of mode- 
rately superheated steam, when the earthy salts remain 
undissolved, and a nearly clear solution of gelatine is 
obtained. These materials, (d) and (e), if carefully prepared, 
are practically equal to the next, they are alike in composi- 
tion, but would be regarded as a ** substitute," or even as an 
adulteration, if supplied in place of the following varieties : — 

(/) Fish gelatine, from the bones, skins, and swimming 
bladders of fish. 

(g) Isinglass, the most expensive variety, derived from the 
swimming bladder of the sturgeon. 

In addition to these are several varieties of more or less 
importance : — 

(h) Liquid glues. 

(i) Soluble and mouth glues. 

(i) Vegetable glues, ** artificial glues,'* or glue substitutes. 

III. Size. — Prepared already in the moist or gelatinous 
state from light-coloured glues or from gelatines by treatment 
with hot water. 

Grades of Glue. 

Besides these broadly distinguished commercial forms, 
the trade recognises a large number of qualities known as 


grades, and separated not always by their value as regards 
physical properties of colour, tenacity, etc., but often by their 
fitness for special purposes. The price, like that of other 
commercial products, is frequently artificial. It is often less 
necessary to prepare a glue that shall possess ideal qualities 
as regards appearance or tests, than to suit the fancy of a 
customer. It has been found even that a glue which will give 
excellent results with one consumer may fail with another 
for the same purpose, on account of some slight difference 
of working. Therefore, it has been customary to keep a book 
with a record of each important client's special requirements, 
the test figures and characters of the article he prefers, and 
memoranda of the details in the process that has obtained it. 
After a time this becomes simplified by the storage of num- 
bered standard specimens, recording particular grades, each 
of which can be immediately referred to the date and par- 
ticulars of manufacture as recorded in the firm's books. By 
this means it is secured that future supplies shall be always 
** up to sample,*' which is often a matter of contract. Exact 
and regular working, strict cleanhness, observance of tem- 
perature and other physical data, and scientific supervision, 
are clearly necessary. **Eule of thumb" is never quite 
certain to produce the same article twice. 

It is often said that the variation in raw materials will 
occasion fluctuations in the quality of the product; this 
to a certain extent in ordinary working is true; but with 
substances such as glue, which depend for their essential 
properties on a chemical entity of practically definite com- 
position like gelatine, there is no reason why the process 
should not attain almost the exactness of a chemical analysis, 
where the sample is accurately averaged, weighed quantities 
are taken, and conditions of temperature, strength and time 
are rigidly observed. 

On a large scale the raw materials, judiciously chosen^ 


will average themselves, and then, if at every point the con- 
ditions are kept intelligently constant, the quantity of the 
finished product may vary, but its quality should be certain, 
the price being effected by the yield. 

Hide glue has the strongest adhesive power, and also 
the highest price ; it is mainly employed by carpenters and 
joiners. The best grade of hide glue is used by cabinet- 
makers ; it must have " great body and light colour, must 
not be too quick in drying, or the joint may crack, nor too 
slow, or the work will be delayed ; it must be sweet, and not 
turn dark over night **. The workshop test is simply that 
of joining two pieces of wood in diflferent ways, and after 
allowing time to set, generally forty-eight hours, trying the 
joint by hand, as there is rarely a testing machine or weights 
available. (See the section on Machine Testing, Chapter V.) 
The result will obviously depend entirely on the skill and 
judgment of the manipulator, and the fairness with which 
the experimental joint is made. In America maple is gener- 
ally used ; in England, mahogany — this should not be new 
or unseasoned. 

For Veneers the "foam test'* (q.v,) is of great im- 
portance, and is usually the first property inquired into. 
It is well known that the strength and soundness of a 
joint is much impaired in all cases by the presence of air 
bubbles. In veneering the glue must not foam when 
applied with revolving brushes. " Veneers glued with such 
stuflP do not stick ; they split off — and if they do not come 
oflf altogether, they show a rough surface full of air 
bubbles." The remark as to one operator getting a good 
result with a glue which does not suit another particu- 
larly applies here, as the fault frequently lies with the 
strength used, or the rapidity of working, rather than 
with the quality of the glue. 

We shall speak elsewhere of the practice of incorporat- 



ing more or less inert powders with the glue in the course 
of manufacture, usually at the end of the boiling, resulting 
in the production of an opaque whitish product. The 
** barytes '* (sulphate of barium), used in some varieties of 
**Eussian glue," is purified from iron, etc., by treatment 
with hydrochloric acid, then ground, carefully levigated 
in water, and mixed by stirrers with the melted glue. 
If not finely divided it is apt to settle, on account of its 
weight; the same is the case with white leady which is 
also used. 

Russian glues may contain 4 to 8 per cent, of barium 
sulphate, white lead, zinc oxide, or chalk. They are met 
with in dirty white short thick cakes. The heavy powders 
add to the weight of the product, ** without injury to their 
adhesive power," according to some authorities, ** with a 
distinct increase," according to others. Experimental tests 
have shown, as was at first probable, that of the variety 
of powders tested, the effect in small quantity was either 
"nil or a slight decrease; in large quantities the product 
was distinctly weaker; in none was there any augmenta- 
tion of strength. They are manufactured to answer the 
demand in certain quarters of the trade, and also more 
especially to keep a certain brand distinct. In the States 
they are included under ** Coloured Glues," and usually con- 
tain oxide of zinc, which addition prevents souring, but is 
apt to cause foaming when used with rotary brushes. 

Commercial glues are usually opalescent, or even may 
be *' muddy," turbid, or very dark brown, like some Scotch 
glues; without inferiority of strength. A dark glue, when 
the colour proceeds from the materials, and not from over- 
heating, will often be stronger than a paler glue which 
has been chemically bleached. Still, for the superior classes 
of work a glue of clear amber, or at most brown-yellow 
colour, is required. It should be free from streaks or spots, 


which show imperfections in the manufacture, dry and 
hard, with a glassy fracture, not too brittle, but some- 
what elastic. 

Cologne glue is found in short thick cakes, very pale 
in colour, but of good strength if properly prepared. It is 
made from scrap hide, or even from some kinds of offal, 
limed, and then cautiously bleached by a solution of chloride 
of lime, using about ^ lb. per cwt. The process is care- 
fully watched, and is stopped when the desired effect is 
produced, as if kept on too long the stock would be rotted 
and the product spoilt. Thorough washing is necessary 
afterwards, and sometimes the use of sulphite of sodium 
as an '*antichlor" to remove any remains of hypochlorite. 
Some manufacturers, after soaking half an hour, add enough 
hydrochloric acid '*to give an acid taste," allow to act for 
a quarter of an hour, and wash. 

Russian glue is a trade name indicating the particular 
kind of opaque article we have described above. It may 
be skin glue or bone glue. Clear glues are also made in 
Russia, the raw material being abundant. A very large 
quantity of horse hides is disposed of, with corresponding 
cUppings and offal, in the Nijnii Novgorod market, with 
also sheep and goat skins from Siberia. 

''Patent glue'' is a title applying to an indefinite num- 
ber of preparations, but specially means a good bone glue, 
in thick cakes of a deep dark brown colour, not showing 
net marks. It is very glossy and swells up much in water. 

" French glues " (the word not always implying that 
they have been made in France) are light-coloured thinnish 
square cakes ; they have been chemically bleached, usually 
with chloride of lime, but in the joinery trade are often 
found stronger and better for work than very dark reputed 
** Scotch glues " of uncertain character. 

" Oilder's glue ** .occurs in very pale yellow thin cakes, 


tied in packages of 1 kilogramme. It is skin glue bleached 
with chloride of Ume, and is only the first runnings from 
the boiler. It is often difficult to dissolve. 

Size glue and parchment glue are also trade names ; 
the latter was once almost exclusively used in bookbinding. 

** Paris glue,** used for sizing, is brown, opaque, and 
almost always ^oft, and is employed by hatters. It is 
intentionally too long boiled, so as to be hygrometric and 
hardly adhesive. 

Fish glue from the skins, bones and swimming bladders^ 
is much used by cabinetmakers, and is said by Wagner, 
when properly made, to be indistinguishable from glue 
made from bones and other animal refuse. 



As to the sources of skin glue the following information 
has been given :- ~" 

Calf and sheep skins yield a superior glue : that from 
horse hides is usually dark and poor in quality, although 
with careful working a strong product can be obtained from 
the latter. 

Tannery waste, i.e., the foot, head and buttock cut oflf 
before tanning, may give 44 to 46 per cent, of glue. Scarf 
skin of bullocks' hides and waste in fleshing the hide, tendons 
and hinder parts of cattle yield from 30 to 35 per cent. ; horse 
sinews 15 to 18. 

Bullock's feet and parchment shavings are the richest 
stock, giving 62 per cent, of their weight of glue. Among 
other materials are : — 

Old gloves, rabbit skins from which the hair has been 
removed by hatters, dog and cat skins, earlaps of sheep 
and cows, sheep's feet with the tendons and small bones, 
waste of skins generally : these may yield 38 to 43 per cent. 
Refuse of tanning and of kid gloves forms a light product, and 
requires no preparation. 

Skins of hogs, hares and rabbits yield a light-coloured 
glue of little consistency, used for sizing and paper. Pig glue 
is largely made in Chicago. 

A considerable number of skins come to the warehouses 

which have been used for packing various articles, such, for 



instance, as indigo from South Africa : they have been so 
knocked about in transit that they are useless for tanning, 
but they will frequently yield 50 to 55 per cent. glue. Waste 
of Buenos Ay res skins may give 60 per cent. 

Cartilages and other waste of fish are usually made into 
gelatine or imitation isinglass. 

Good stock should yield 50 per cent. glue. Poor may be 
as low as 20 per cent. Stock before drying is said to be 
'* green,*' and is stored during the winter in wooden vats or 
in cemented brick tanks filled with lime water about half 
saturated and covered up from the air. As to judging of glue 
stock, some valuable notes are given in a work on glue lately 
published by the American Provision Company, Chicago. 

** Dry uncured or salted stock, such as raw hide or South 
American, if soaked for twelve hours in cold water gains about 
50 per cent, in weight, and still remains tough, and the water 
sweet. The moisture, dirt and salt should not be over 10 
per centv 

** Green salted stock, such as hide pieces, sinews, calf 
heads and pates should have no excess of salt, nor be foul, 
discoloured, or heated ; should be tough, with the hair not 
loose, and have a mild animal odour. Moisture and salt not 
over 40 per cent. 

"Dry limed stock soaked twelve hours develops a charac- 
teristic odour, and should be firm, fibrous, and have no slimy 
pieces. The water should not be dark. Lime, sand and 
dirt not over 5 per cent. 

** ^reen limed stock' should be smooth and soft, any 

r ■ 

remaining hair being easily detachable, while the liquor 
should be fairly clean, sweet, and not too alkaline. 

'* A large quantity of waste bones are accumulated in the 
preparation of tinned provisions ; if these have not been 
overheated, and are in good condition, a considerable amount 
of glue can be obtained from them, the bones of the head, 


ribs, and feet giving a better yield than those of the thighs 
and legs. 

** Horn piths should not contain over 12 per cent, mois- 
ture, and should not have been overheated in drying ; they 
should have been cleansed from skin and hair, which are of 
little value to the glue maker. 

'* The age of the animals yielding glue stock has an 
important influence on the product. While from younger 
animals the product, as a rule, is of lighter colour, more 
abundant, and more easily obtained, it contains more chon- 
drin, so, that for solutions of equal strengths, those fron^ 
mature animals will be found to be of the greater consistence 
and the glue more soUd» 

''Abroad, dry hides are often, for weighing, soaked in 
' chlobarium,' a solution of barium chloride, and then in a 
bath of dilute sulphuric acid, IJ per cent., which readily 
soaks in, combines with the barium to form the white 
insoluble powder of barium sulphate, leaving weak hydro- 
chloric acid in the fibre, to be afterwards neutralised in liming, 
the chloride of calcium dissolving out. This treatment affects 
considerably the subsequent making of glue, as, besides the 
effects of the acid, the sulphate of barium vsdll render the 
liquors cloudy, and be difficult to clarify. Of course if 
* coloured ' glues are to be made this will be no detriment." 

Skins are composed of a number of elements of which 
the most important are: — 

1. Scarf skm or epidermis. Thin, horny, and mainly 
composed of keratin. It is scraped off before tanning, and 
does not enter into the preparation of glue. It carries the 
hairs, accompanied by fat and ^weat glandsw 

2. The corium or true skin, ^chiefly composed of interla- 
cing white fibres of connective tissue (95 per cent.) embedded 
in a structureless matrix of coriin.. In the operation of 
Uming, the coriin is in part dissolved and the fibres loosened, 


hence the softening effect, which should not be allowed to 
proceed too far, or waste will be occasioned. Under the 
action of hot water the corium is gradually but entirely dis- 
solved, and converted into gelatine — only the hairs, glands, 
vessels, nerves, muscular fibres and a little yellow elastic 
tissue are left, and form a small quantity of sediment. The 
microscopic examination of glue stock is often of great value. 
It is conducted by soaking for some days in alcohol, and 
cutting very thin sections with a razor ; these are examined 
under the microscope with and without staining, or soaking 
in glacial acetic acid and glycerine. 

Keratin is the nitrogenous horny substance constituting 
hair, epidermis, horns, hoofs, feathers and nails. It is very 
diflScult to dissolve in either hot water or acids, but on pro- 
tracted heating it gives a solution which does not gelatinise ; 
consequently the forms of keratin possess no value for glue 
making, and have to be removed as far as possible. They 
are generally used up in the manufacture of ferrocyanide of 
potassium (yellow prussiate), in the manure or other trades, 
among which may be mentioned the use of a large quantity 
of hair in plastering, and of horns and hoofs for handles, etc. 

It has been asserted that the bones, and other parts of 
birds and reptiles, jdeld a superior gelatine. I have not found 
commercial confirmation; moreover, the supply would be 

Bones contain about one-third of their weight of animal 
matter, which is almost entirely convertible into glue. The 
soft bones of the head, shoulders, ribs, legs and breast, and 
especially deer's horns, and the bony core of the horns of 
horned cattle, yield a larger quantity of glue than the hard 
thigh bones and the thick parts of the vertebrae, which are 
principally composed of calcium phosphate, and require a 
more prolonged treatment to extract the glue-making cqa- 
stituents. Buckshom, the osseous nucleus, is reckoned a 


superior stock: it contains very little fat, and requires no 
steaming before liming. 

The upper thigh bones are used for making piano keys, 
handles, etc., while hoofs and horns are separated, as they 
yield no glue (see Keratin), and are used for other purposes. 

Fish glue, from the skins and bones of fish, has very little 
colour and generally less tenacity than hide or bone. It 
always retains a fishy odour, but is preferred for making 
some cements and liquid glues, and for other commercial 
purposes. Sole skins are much used. Carefully deodorised 
by a small' quantity of chlorine, and decolorised by animal 
charcoal, fish offal is made into gelatine or imitation isinglass. 

So-called glues are prepared from various proteid sub- 
stances; in Chapter IV. a few of them are described. 

Vegetable matters have also been used, principally those 
containing pectin or vegetable jelly. Under the name of 
** Vegetable glue,*' Frank Hepburn has patented a glue or 
sizing obtained from the palmetto or similar plants. It con- 
sists largely of pectinous matter. The roots or underground 
stems are disintegrated and steamed at 300° F. for an hour, 
then boiled with water. The liquid is clarified and evaporated 
in vacuo to the required density. It is said to possess con- 
siderable viscosity, to be tenacious, and when set, to be Uttle 
affected by damp or liquids. 

Various gelatinous bodies derived from seaweeds will be 
described in Chapter IV. 


The preliminary liming process often constitutes a 
business quite separate from that of the glue boiler, so that 
the stock is purified before being sent to the glue manu- 
facturer, who again; however, treats it with weak milk of 
hme followed by thorough washing previous to boiling. 
Bones are sometimes steamed for a few hours to soften 


them before liming. Foot-bones and buckshorn are not 
steamed : they are left in the lime vat eight to fourteen days. 

Besides the " collagen/* or matter that in boiling yields 
gelatine or glue, there are a number of other substances 
present in the heterogeneous mixture known as ** glue 
stock". Even, after removal of fat by steaming, ethereal 
solvents or alkalies, only a portion of the residue is con- 
vertible into glue. In the dermis at the back of the scarf 
skin a quantity of mucin is frequently found ; this is soluble 
in dilute alkalies, and possesses no gelatinising power. The 
same is the case with the remaining blood, gummy and 
extractive matters, which would darken the colour and 
convey no strength, besides increasing the tendency to 
putrefactive change. < To dissolve these, at the same time 
softening and loosening the tissue, is the object of soaking 
in lime water or "liming". If the fat has not been previ- 
ously removed, it also is acted on more or less rapidly by 
the lime, being ** saponified " or converted into calcium 
salts of the fatty acids, so-called lime soaps, wljich generally 
detach themselves, and float as a curdy scum on the soak. 

Careless liming will produce a fat running high in free 
fatty acids, which means for each 13 per cent, o'f free fatty 
acids a loss of 1 per cent, grease on account of the liberated 
glycerine. The safest way to prevent loss of fat is to trim 
the stock carefully of all fatty tissue, and to render such 
trimmings separately. 

The liming and drying are generally carried out in the 
tanyard. If not carefully done by the tanner, the stock is 
of little value to the glue manufacturer. If the stock lies 
too long in a heap, especially in summer, it becomes putrid, 
and the value is not restored by hming. On the other hand, 
it has been stated that it is sometimes advantageous to allow 
the offal to become somewhat decomposed, and then to 
disinfect with chlorine or sulphurous acid, the idea being 




that extraneous mattery are more easily removed. Such a 
proceeding is very dangerous. If badly dried after Uming, 
the stock becomes mouldy. Drying in the open air is rarely 
practised, and then only in the spring and autumn, as, if the 
stock is allowed to freeze, it appears that the fibre is so 
injured that it yields glue of very little consistency (Brannt) ; 
in the summer the stock is liable to putrefaction. During 
liming the hair is loosened, and is afterwards easily scraped off. 
Eotary vessels save considerable time and render the process 
more regular. The proportion of lime liquor to stock should 
be such in ordinary vats that there is room for stirring and 
circulation. It is better to have excess of liquid than too 
much solid material. It has been remarked elsewhere that 
the liquid of the liming vats should not become very dark 
coloured, ammoniacal or putrid ; each pf these faults indi- 
cating a loss of glue substance, and a deterioration of the 
product. An American authority asserts that ** every pound- 
of ammonia escaping is equivalent to twenty pounds of glue 
' stock lost ". This is probably an overstatement, but the 
actual ratio of ammonia to dry gelatine is about 1 to 6, 
so that a large loss, both in quantity and quality, occurs 
if the liming be not properly conducted* A great part of 
the care that is necessary is to control the action of the 
innumerable bacteria — mostly micrococci — which may ob- 
tain access to the liming liquid. The lime itself acts as 
a mild antiseptic. Pettenkofer held that it ** destroyed 
rapidly and completely the organisms of putrefaction," but 
we now know that it simply modifies, differentiates, or 
retards their action. The well-known use of lime water 
for preserving eggs depends on the deposition of carbonate of 
lime in the pores of the shell, rendering it impervious, and 
also on the lime coagulating the albuminous envelope of 
the egg. In the same way, in the Kming vat, the surface 
of the liquid becomes continually coated over, by the action 


of the carbonic acid of the air, with a film of carbonate 
of lime which is fairly coherent, as shown by touching it ; 
and the air, and -particularly the bacteria, are thereby to 
a great extent excluded. The lime coating on the pieces 
themselves is also a protective. 

Stock, which has become soft or discoloured, may some- 
times be restored by steeping for twenty-four hours in clean 
water, washing, again soaking in lime water for some days, 
washing again, draining, and drying at a moderate heat. 

As slaked lime, or calcium hydrate, is so slightly soluble 
in water — 1 part requiring 700 parts for solution — it is 
customary to employ **milk of lime," or lime suspended in 
water, for the first soaking, and lime water or solution of lime 
ior the subsequent ouqs. Many samples of so-called milk of 
lime that we have examined have been quite valueless, having 
become carbonated, or a bad quality of quicklime having been 
originally employed. It must be remembered that only the 
hydrate of lime which is present in solution in lime water is 
of use, whereas in *' milk of lime " so much carbonate and 
other impurities may be present that the liquid, though thick, 
may be quite useless. The value of a lime should always be 
tested by determining the amount of real calcium hydroxide, 
Ca(0H)2, contained in it. The operation is an easy one, and 
is thus conducted : Water, free from carbonic acid, is first 
prepared by boiling distilled water for half an hour in a 
strong round-bottomed Bohemian or Jena flask. While 
steam is still issuing, the flask is removed for an instant, 
closed by a well-fitting greased cork or a caoutchouc stopper, 
and allowed to cool. When the temperature has somewhat 
fallen, the cooling may be cautiously accelerated by dipping 
into a pail of warm water, then transferring to the cold stream 
from a tap. The water may be preserved in the flask, or, 
preferably, a number of bottles with vaselined stoppers should 
be filled quite full and retained for use. 


From the sample of lime, well mixed, a small portion 
(about 0*25 gramme) should be accurately and rapidly weighed^ 
placed in ar -wide-mouthed stoppered bottle holding about 300 
cubic centimetres, 250 cc. of the boiled water added, and the 
whole vigorously shaken at intervals for one hour, and then 
allowed to settle. The whole of the calcium hydrate will 
now have dissolved. Fifty cc. of the clear liquid should now 
be withdrawn by a pipette, transferred to a flask, coloured 
with an indicator (either phenol-phthalpin, methyl-orange or 
litmus may be used), and its alkalinity determined by running 
in decinormal hydrochloric or sulphuric acid from a burette 
till the change of colour occurs. Each cubic centimetre of 
the acid corresponds to '0028 gramme of calcium oxide, or 
•0037 gramme of the hydrate, Ca(0H)2. The amount by 
calculation will give the percentage of real lime present in 
the sample. It is well to notice that any soda or potash 
present will equally neutralise the acid, and be returned as 
lime, but as these are of almost equal efficiency their presence 
in small quantity has no disadvantage. For special work it 
will be necessary to have a full analysis. As a rule the pro- 
duct made from limestone, or " stone lime,** is the best article 
in commerce, and is much more free from stones and clay 
than ** grey lime" or "shell lime". The best stone lime 
contains sometimes only J per cent, of impurities, and seldom 
more than 5 per cent., while inferior kinds of grey lime may 
contain as much as 60 per cent., and would be of little use in 
glue making. 

A good sample of quicklime having been obtained, it is 
slaked, and the powder of calcium hydrate removed without 
delay to closed wooden mixing vats, where it is stirred by 
mechanical agitators with the requisite quantity of a soft 
water. It is a great mistake to use too much lime : the 
strength in real lime should be 1 to 2 per cent, of Ca(0H)2, 
and can be calculated from the result of the alkalinity test. 


Thus, if the content of calcium hydrate have been found to be 
90 per cent., it will be necessary to use 11 parts of the 
crude lime to 1,000 parts of distilled water, or 770 grains per 
gallon. But as soft, or softened, water, containing some 
carbonic acid and carbonate, and therefore occasioning a 
certain amount of waste, must for economy be employed, 
it may be laid down that about two ounces of good lin\e. per 
gallon will yield a milk of lime of suflScient strengthMit 
should always be thoroughly mixed, and strained from lumps, 
as these occasion spots in the stock, with subsequent faults in 
the glue, and waste of acid if the sulphurous process be used. 
The practice of commencing with a weaker milk of Ume, and 
increasing the strength for the second and third soakings, 
although adopted in a numiber of works, particularly in 
America, is not so good as that of treating first with lime 
milk properly prepared in the way we have recommended, 
and then with clear lime water of maximum strength 

Both milk of lime and lime water must be protected from 
the air. The latter should be clear and colourless, and its 
strength should be periodically tested as aboVB. It is obvious, 
however, that a saturated lime water can always be decanted 
from a milk-of-lime vat that has been stirred and allowed to 
settle. The solubility of lime at various temperatures is given 
as follows in grains per gallon ;: — 

Temp. C. 

Temp. F. 

Grns. CaO. 

Grns. Ca(0H)2- 













the solubility decreasing as the temperature rises. 

Filter Presses for Extraction^ Washing, and Clarifying, 

For nearly all purposes where an insoluble precipitate 
has to be separated from a liquid, or a liquid has to be made 
clear, the old-fashioned process of sedimentation in vats and 


decantation of the liquor has been replaced wherever possible 
by the use of filter pressea (Pig. 1) for the following reasons : — 
1. In many cases, when the precipitates are light and 
flocculent and easily raised by vibration, convection currents 
or entangled bubbles of air, the process of deposition takes a 
considerable time, during which a great amount of floor 
room and plant is monopohsed. Some precipitates absolutely 
refuse to settle clear within a reasonable time vrithout the 
use of heat or of a clarifying agent, involving additional 
expense ; although the precipitate be naturally heavy, it is so 
finely divided as to be intractable, like the familiar case of 
sulphate of barium. 

2. During the time required, detrimental changes are apt 
to arise in the liquid, even when carefully protected, or if (at '. 
further expense) a preservative like borax or salicylic acid be 
used. This is particularly the case in the glue trade. 

3. It is impossible to draw off more than a varying 
fraction of the liquid from the precipitate, a proportion 
depending on the care devoted to the operation, and to the 
nature of the precipitate, and circumstances connected with 
quietness, density and temperature. The residue is a sludge 
of precipitate and liquid ; the option of further treatment 
being decided according to the value of the materials be- 


(a) Washing by decantation ; that is, flushing with 
further quantities of water, mixing, subsidence, and syphon- 
ing or tapping ofif as before ; this would require to be re- 
peated several times, and the washing would never be com- 
plete. A very large volume of wash water is also required. 

(6) Discharging, if the mixture will not pay for further 
treatment; with the accompanying difficulties of disposal 
owing to odours, river pollution, etc., besides the waste of 

(c) Drying by heat or burning^ with a consumption of 
fuel corresponding to the amount of water remaining, and 
the same waste of matter. 

On the other hand a filter-press can, except in very rare 
cases, produce a clear liquid and a nearly solid Cake with 
considerable rapidity, under a pressure varying with the 
character of the work, and with a great economy of space. 
The liquid is ready for an immediate continuation of the 
process in hand, while the cake can be either soft-pressed and 
washed, or if valueless, hard-pressed and dealt with further in 
the ways indicated above, with an extra facility as containing 
less water. It may be remarked that with the many modern 
improvements in saving waste products, it is very seldom that 
residues do not admit of being worked up to some advantage. 

Filter presses are usually constructed with an iron frame- 
work on account of the greater strength. But where, as in 
the preparation of the better classes of size, or of gelatine, 
there is danger that accidental rusting or action of acids on 
the iron might occasion spots or general discoloration, 
filter presses of wood are available, and have been of late 
greatly improved. It is absolutely necessary that the wood 
should be well seasoned. When first used, even the best 
woods give off more or less tannin in washing and cleansing, 
so that the press is not really up to perfect efficiency till this 
has been extracted and washed out ; then the press should 


not be allowed to rest idle and dry, or it may crack or warp, 
nor to remain in contact with foul water, so as to develop 
moulds, green algae, or offensive bacteria, which may each of 
them, in subsequent working, cause faults in the glue. A 
press, particularly a wooden one, should always be kept 
scrupulously clean ; if it is absolutely necessary for the wood 
to remain at rest for a time, it must be washed with dilute 
chlorine water, or very weak chloride of lime, then with 
clean good water and allowed to dry, but it is rare for a leak 
not to occur on resuming working. The filter chambers are 
now made recessed from a solid slab, and hence more durable 
than framed plates. 

Water Supply, 

In glue making, as in other manufactures, one of the 
first necessities is to have at Command an abundant and 
regular supply of water. When starting a new factory in the 
colonies, where great facilities for economic production of 
glue often occur, owing to the cheapness and abundance of 
the raw material — which is often a by-product, though an 
important one, of the production of hides; hair, horn, and 
meat, either frozen or canned — failure has frequently re- 
sulted from a surprising inattention to this first requisite. 
The main features to be considered are : — 

1. Abundance and constancy of the supply, — A very 
large quantity of water is wanted, even in a small factory, 
for the liquors, boilers, and especially for cleansing and 
washing. If this last item is stinted, the factory, as a 
whole, is apt to get ** sour " or ** foul," owing to the inroads 
of bacteria and moulds, and it will become almost im- 
possible to turn out a good or marketable article. There- 
fore, the rainfall of the district must be taken into account, 
as if very irregular, storage in reservoirs will be necessary. 

2. Quality. — The water of rivers is often plentiful and 


soft, but turbid, discoloured, and organically contaminated. 
That of wells is frequently clear and colourless, but unduly 
hard. Bain water is very soft, but can rarely be collected 
in sufficient quantities. A good water for manufacturing 
purposes should be reasonably soft — i.e., free from lime 
and magnesia salts beyond about 10 to 15 grains per 
gallon = 14 to 20 parts per 100,000 — not containing an ex- 
cessive amount of mineral salts, especially carbonate of 
soda, free from iron, also from sulphuretted hydrogen or 
more than traces of free ammonia. The presence of salt 
(chloride of sodium) is considered objectionable in glue 
making, as it has a liquefying quality. 

The waste occasioned by hard water for boiler feeds 
is common to all manufactures. The effect of the deposit 
known as ** scale " has been variously stated, but is always 
large. Approximately, the loss of heat increases as the 
-square of the thickness of coating, thus if yV iiich scale 
•causes a waste of J, a hard scale J inch thick will result 
in a loss of nearly 50 per cent. 

For liming and cooking, the permanent hardness due to 
chlorides and sulphates of lime and magnesia does not 
interfere so much with the glue processes, as these earthy 
salts are not deposited on boiling. For this reason it has 
actually been 'proposed that water with very high tem- 
porary hardness, due to earthy bicarbonates, should be 
** acidified " — better ** neutralised " — with sulphuric or hydro- 
chloric acid to turn the carbonates into chlorides oi* 
sulphates. But excess of these salts, particularly of 
chloride and sulphate of magnesium, fixes the earthy base 
in the fibre of the stock, occasioning ''pasting up," with 
a loss of solubility to the gelatigenous matter. 

Failing a proper supply the chance of remunerative 
working will be diminished. Difficulties, however, may 
often be averted by — 


1. Artificial softening, as in Clark's, the Atkins, Wright's 
or Archbutt-Deeley process (vide Water Purificatioriy Eideal, 
1897, Chapter X., ** The Softening of Water "). 

2. Sand filtration, or such apparatus as the Hyatt, 
National, or other filters, which are worked by pressure 
and are much used in the States. It is always advisable 
to have periodic analyses made of the water. 

Washing LiTned Stock, 

The excess of lime, salts, and dirt, must be thoroughly 
removed by repeated washing with water. As it is wasteful 
and injurious to unduly protract this process, it is necessary 
to test at intervals the liquors coming away. As soon as 
the impurities have been removed, the wash waters will 
be colourless, neutral, and on evaporating about 20 cc. on 
a water bath, will leave only the natural water residue, 
not blackening nor giving an animal odour on burning. 
The simplest plan is to place the limed stock in nets or 
wicker baskets suspended in running water, but this ele- 
mentary method is open to several disadvantages : — 

1. It fouls a large volume of water, and may lead to 
legal interference. 

2. The very simplicity is apt to lead to neglect of 
precautions such as brushing away solid particles of lime 
or softened animal matters. 

3. It is not so easy to test the liquors. 

4. The great quantities of water carry ofiF small pieces 
of glue stock and gum fat, if catch basins are not provided 
suflSciently large to allow fat, glue stock, hair and lime to 
separate from the water : they require frequent cleaning. 

It is well known that small quantities of liquid, fre- 
quently renewed, and thoroughly drained off each time, 
effect the most complete and economical washing, and in 
the shortest time, therefore pits or vats with proper arrange- 


ments for stirring, draining, and inspection are in every 
way preferable. . The lime scum from the pits is often used 
in the manufacture of fertilisersi 

To remove the embedded lime, dilute hydrochloric acid 
(1 or 2 per cent.) is sometimes used, followed by plain water 
as usual. But in this way fatty acids from the lime soaps 
are lost, while a careless use of acid allows a great deal of 
raw gelatine to be dissolved and carried away, especially in 
horse fleshings. 

On the other hand, if not washed properly free from lime, 
ammonia may be evolved in the boiling.^ "Weak sulphurous 
acid has been used with advantage. 

Hoeveler's glue stock washer (American patent) is de- 
signed to remove lime without breaking the material by 
means of a paddle wheel with' spoon-like arms, with a 
settling tank to gather any particles ; the stock can then 
be dried with the expenditure of very little water for 

Washing Salted Stock 

must be efficiently performed, as the presence of salt deterio- 
rates the quality and appearance of glue. Salted stock is 
not used in the manufacture of size. The progress of 
washing should be observed by testing the liquors at inter- 
vals with nitrate of silver in comparison with the water 
supply ; the density of the white precipitate, or cloud, of 
silver chloride will reveal when the process is finished : this 
should be done after each portion of fresh water has had 
time to soak in, say fifteen to thirty minutes after adding. 
Washing water Is frequently warmed to 80° F. : some use 
the waste water of vacuum pans at 90° .to 98° F. 

Use of Soda in Glue Making, 

"When a sheet of gelatine is immersed in dilute soda the 
solution absorbed by the gelatine contains more soda than 


that left behind in the vessel, the amount absorbed being 
greater with dilute solutions than with strong ones. The 
same is not the case with common salt. Hence, when treat- 
ing with soda to remove fat, it must be remembered that the 
product will retain alkali, which can only be imperfectly 
removed by washing with water, and may possibly cause 
injury in the subsequent heating. Soda, therefore, has to be 
used with caution, and usually only as a supplement to the 
milder alkali lime. 

Fleck, however, considers it better to employ a weak 
alkaline ley, 2 parts soda and 3 quicklime to 300 to 400 

Milligan, of New Jersey, U.S.A. (English patent, 3393, 
1888), washes fresh pieces and fleshings in clean cold water, 
places them in a basket and plunges into boiling water for less 
than one minute. This causes the stock to shrink, hardens the 
tissues and diminishes the weight by over 40 per cent. The 
stock is now dried in a current of air at 125° to 170"^ F., then 
again plunged for ten minutes into a boiling solution of 
12 lb. of soda crystals in 100 gallons of soft water to remove 
fat, washed with hot, then with cold water, and soaked till 
hard in 1 per cent, alum solution. Finally it is converted 
into glue in the ordinary way* 

Sulphide of Sodium is sometimes used along with lime 
in the preparation of skins : the stock will usually then show 
brown, bluish or green stains, due partially to compounds of 
iron. Treatment with weak acid (hydrochloric of J to 1 per 
cent.) will frequently effect improvement. Careful washing 
is necessary afterwards, but the sulphur retained in the glue 
will often occasion trouble. 

Glue from Leather Waste. 

The fat is eliminated by boiling with soap and soda, and 
the tannin is extracted by milk of hme or other alkalies. 


By contdnuOTiB wEhshing the alkalJDe tannate is removed, " the 
leather becomes hide, and is boiled to glue. The yield of 
glue from antamied hide being about one-third, that from 
tanned leather ia one-quarter." As the retention of the 
smallest quantity of tannin prevents the animal tissue from 
dissolving in water, the operation is rendered quicker and 
easier by reducing the leather scraps to a coarse pulp in a 

FiQ. 2. Hollander, 
machine similar to the rag engine (Fig. 2) or " Hollander " 
used by paper makers, and then heating in a pressure-boiler 
under a pressure of two atmospheres vi^ith 15 per cent, of 
slaked hme. 

Action of Bacteria in Salting and Liming. 

The materials for the manufacture of glue — the " glue 

stock " — are rarely obtained in their fresh state ; they have 

asually been salted, limed, or dried for their preservation. A. 

paper by F. H. Haenlein on " The Action of Salt on the 


Putrefying Bacteria of Hides" (Dingier 's Polyt. J., celxxxviii., 
214), gives an interesting investigation of the influence of salt 
(chloride of sodium) both in softening and preventing fouling. 
Haenlein soaked cleaned pieces of hide in sterile water, and in 
saturated, 10 per cent, and 2 per cent, solutions of salt, for 
seven days, in vessels exposed to air. At the end of the time 
Qne-tenth cubic centimetre of the solutions was incubated on 
a gelatine plate. The saturated and 10 per cent, developed 
only one colony, while the 2 per cent, gave 871, mostly 
micrococci and non-liquefying : the water alone showed 
" colonies innumerable, including micrococci, a large number 
liquefying ". 

A second series, incubated for eight weeks in closed 
vessels, gave in the saturated and 10 per cent, solutions 
no colonies, in the 2 per cent, many colonies but none hque- 
fying, in the water a large number of liquefying bacteria. 
In the two latter cases the hide was discoloured and soft and 
the liquid putrescent ; in the stronger solutions little visible 
change had occurred beyond a slight hardening. The alka- 
linity of the 2 per cent, solution was always stronger than 
that of the water. 

It must be remembered that the salt dissolves a variable 
quantity of substance which would be available for glue 
making, therefore reducing the yield from the clippings and 
waste ; also that much washing is required if a concentrated 
brine has been used, as the presence of an undue amount of 
salt is very objectionable in glue. 


Several patents are extant for the prevention of putre- 
faction during the soaking processes by the addition of 
preservatives. Only such agents are available as are cheap, 
on account of the large volume of hquid ; they must also 
not show any tendency to harden the membrane or combine 


with it, therefore the selection is limited. Dark-coloured 
tar products are inadmissible ; metallic compounds, such as 
mercuric chloride, chlorine and hypochlorites are excluded ; 
formaldehyde in weak solution (1 in 10-to 100,000) has been 
used beneficially. In this small quantity it does not harden 
the stock, nor affect the subsequent boiling, as it is dissipated 
by the heat. Boric acid, and its preparations, in spite of 
their low antiseptic power, have been much in favour. 
Phenol is hardening, and gives its odour to the glue. The 
minimum strength required to prevent putrefaction for a 
reasonable time is : — 

Phenol (carbolic acid) 1 in 1000. 

Boric acid, about . 1 in 200. 

Formaldehyde, about . . 1 in 20,000. 

Disinfect unh of Hides. 

In the United States, and some other countries, the 
departments of agriculture require hides of neat cattle that 
have not been salted or arsenic-cured, to be disinfected by 
sulphurous acid, carbolic or corrosive sublimate (mercuric 
chloride). Under certain circumstances such treatment 
exercises a very injurious influence on the glue made from 

Extraction, coonnionly called ** Cooking ". 

^he earliest mode of making glue was to place the stock 
in a vessel, cover it with water, and place it over a naked 
fire, but the product was very liable to be dark coloured and 
of inferior quality through overheating. An improvement 
was to support the materials on a perforated grid a little 
distance above the bottom, so as to save them from risk of 
burning. The water should be gently simmered with agita- 
tion. Then the first liquor is run off, fresh water added,, and 
the heating continued. The extraction is repeated until the 


solution ceases to gelatinise on cooling. The liquors give 
glues of different grades, the first being the best. 

Fractionated boiling. At first a comparatively small 
quantity of water is added to the materials, the cover is put 
on, and the whole boiled about two hours, and the process 
repeated as described above as long as anything valuable is 
extracted. The last liquor is kept for use instead of water 
in a subsequent operation, or is worked up separately. By 
this method injury by heat is avoided, and the product is 

Sigh-pressure steam gives a very strong solution in a 
much shorter time. Its use is more common in England 
than on the Continent. « ' 

Allow to settle warm, a little powdered alum being some- 
times added. 

Eun into wooden moulds, wider at the top than at the 
bottom. When sufficiently firm, . detach from the moulds 
by a sharp knife, and turn on to a damped table. 
The block is then cut into cakes of the required size by 
means of a wire, or by a frame containing a number of such 
wires fixed parallel at the proper distances apart. 

It has been said that it is advantageous to allow the 
offal to become somewhat decomposed, then disinfect with 
chlorine or sulphurous acid. We are unable to say whether 
this is correct. 

With reference to the plant employed, four principal 
systems are in vogue : — 

I. The Open Process, — Here the heating is carried on 
exposed to the air in large pans made of cast, or better, 
of wrought iron (cast-iron pans are cheaper, and rather less 
subject to corrosion, but are liable to fracture from jars or 
abrupt changes of temperature). The heat may be applied 
in five different ways : — 

(a) By an Open Fire. — For the reasons already stated 


this method is objectionable, and is now, except on a small 
scale, almost disused. 

(6) In a Water Bath. — Several large makers treat their 
materials, with sufficient water to cover them, in large 
"coppers," made of wrought iron, immersed in an outer 
vessel containing water, the whole built into a furnace. 
Every one has observed that in boiling glue in the open air, 
unless it is continually stirred, a skin forms, and if allowed 
to pass into the glue would c^use those streaks and clouds 
that are visible in glue cakes that have been improperly 
made. In this process the skin is dipped out at intervals by 
large ladles, and is placed finally, with the undissolved 

Fio. 3. Jacketed Kettle. 
remains of the offtil, on a wire strainer, which retains the 
sohds, to be returned afterwards to the copper for a second 
boil-up vrith water, and allows the liquid to pass to a clari- 
fying tank. 

(c) By a Steam Jacket.- — The pan is made with hollow 
walls, strengthened by straps and stays, and steam at any 
desired temperature is passed through. A jacketed cast- 
iron kettle of this description is shown in the illustration 
(Fig. 3). 

(d) Steam Coils are used in many processes of manu- 
facture for evaporation and internal heating of a body of 
liquid. In the glue manufacture they do not seem to have 
been so successful, on account of (1) the waste of energy by 


the friction of the steam in the coil, (2) their cost, (3) leakage 
of joints. 

(e) Forcing Raw Steam into the Liquid. — The steam 
must be superheated or condensation will occur in practice. 
The temperature is apt to be uncertain, and the results 

Bone glue, made by the open process, is said to be of fair 
appearance, the first runs being equal to the last runs of 
hide stock, to be easy to clarify, but to have a very strong 
animal smell, rendering it unsuitable for certain purposes. 
It is much better if the stock is previously bleached by 
sulphur dioxide, which does not injure the oils and fats. 

Bertram's Process (English patent, 951, 1892) is distin- 
guished by the low temperature used in extraction. The 
stock is ** cooked" at 160° to 170° F. (71° to 77° C.) in a 
jacketed kettle or water bath. It makes very good glue, 
even from sheep stock* 

II. Closed Tanks with Water under 10 to 15 lb. Pressure. 
— Usually two runs are made after the pressure of steam has 
been reduced, each of two or three hours duration. The 
liquors are more concentrated, and therefore require less 
evaporation ; the glues, if carefully made, are equal to those^ 
of the first process, and do not smell. While open vat 
cooking produces about 5 to 10 per cent, glue, pressure 
cooking extracts 10 to 13 per cent. The bone meal yields 
rather less ammonia (2 to 2^ per cent, as against 3 to 4 
by the open method) on account of a certain loss occasioned 
by the greater heat, but more phosphate (60 to 70 instead of 
60 to 60). 

III. French Process. — The bones without water are 
heated by steam at 20 lb. in revolving or oscillating tanks. 
The glue liquor formed by the condensed steam is drawn off 
constantly as soon as produced. After three or four hours 
the bones are taken out, crushed, and boiled up further in 


Open vats. The glue is said to be of best quality, sweet, and 
equal to any fleshing glue. The bone meal shows 2 to 3 
per cent, ammonia, and 50 to 55 per cent phosphate. (See 

Fig. 4. Boiler for Olue Extraction. 

rV. English Process. — The bones are included in a 
steam-tight cylinder and water made to trickle slowly over 
them whilst steam is admitted from below. So the bones 
are digested with the minimum of water, and the solution 


issues from below in a slightly turbid but nearly colourless 
state, and only requires clarifying to produce a superior bone 
gelatine, or at least a high grade of glue. Although some- 
what slow, and requiring careful management, this process 
seems to be one of the best, and can easily be applied to hide 
scraps or other ra^ materials. A number of patented 
apparatus of the kind have a series of perforated alternating 
shelves or trays. 

The following are other processes : — 

Huefs Process for Manufacturing Gelatine and Glue. 

According to the German patent, 19211, 1881, the refuse 
used is brought in contact with a solution of aluminium 
chloride of 2*5° to 5° B. for twenty-four hours, and then 
kept in heaps till used. On boiling this treated material, 
all the fat collects on the surface, whereas, in the ordinary 
treatment of milk of lime, 5 to 7 per cent, of fat is lost. 
- In the English patents, 5249, 1881, and 134, 1882, the 
skin of the fatty animal refuse is first broken up, and the 
crushed material macerated in a tank containing a solution 
of aluminium chloride of about 10° B. instead of lime. If 
the refuse contains much fleshy matter, it will require a 
previous washing, and the chloride bath must be renewed 
at the end of each operation. After remaining in the liquid 
for twenty-four hours, the pulp is taken out, drained and 
submitted to pressure to remove excess of tanning liquid. 
This treatment secures the crude materials from decom- 
position on storage. To separate the grease, a small quan- 
tity (1 to 2 per cent.) of chopped straw is added to facilitate 
the flow of the fat, and the materials are then spread upon 
an inclined hollow plate, heated by steam to a temperature 
not exceeding 100° C, in a layer 6 to 8 inches thick. They 
are then submitted to a pressure of 1 cwt. per square 
yard by another steam-heated hollow plate. Pure fat 


escapes first, and then gelatine and water. The residue 
is placed in a hot press to extract the last remnants of 
fatty and gelatinous matter, and the resulting cakes may 
be employed as food for cattle or as manure. 

Process for Preparing Glue from *' Fish, Whale, and other 
Sea Animals'' (L. A. Groth, London, English patent, 
5,786, 1882.) 

Soak the fish or parts of the same in warm water, 
mixed with acetic acid or vinegar, after first freeing the 
material from all the albumen, salts, and other matters 
capable of extraction by cold water. 

G. W. von Nawrocki, Berlin, patents an apparatus for 
extracting glue from bones (English patent, 5841, 1884) 
successively in the same extracting vessel, by (1) a solvent 
such as benzene for the extraction of all fat, the solvent 
being recovered by distillation, and used several times ; 
(2) a weak solution of oxalic or hydrochloric acid for a 
greater or less time, and under more or less pressure 
according to the age of the bones; (3) hot water for the 
extraction of the glue. A special feature is a perforated 
coil or rose in the top of the extractor employed in 
conjunction vdth an injector below, the effect of which is to 
draw off the solvent from below, and flush the bones with 
it from above. There is also a special method of connect- 
ing the extractor with the condenser, the solvent store, 
and the still in which the solvent is vaporised. 

Buttner's apparatus ' (English patent, 2,615, 1887) is in- 
tended for ** extracting, washing and condensing glue, oils, 
and other substances from bones ". Three separate modi- 
fications for extracting by volatile solvents are described, 
with special precautions for the complete recovery of the 

Bisulphide of carbon, CSg, is sometimes used for re- 


moving grease, but although a good and cheap solvent, 
its volatility and inflammabihty render it dangerous. 

When steam direct from the boilers is used for extract- 
ing glue, it must be ** clean '* ; that is, free from matters 
capable of injuring the product, such as sulphuretted 
hydrogen, ammonia, oils, or empyreumatic substances. 

The capacity required in boiling is stated to be " one- 
tenth more in gallons than the lbs. of stock," i.e., 1 lb. 
stock to 11 lbs. water. Hot water is run in in the follow- 
ing proportions : — 

To 250 lbs. wet stock add 125 gals, water. 
To 100 lbs. wet stock add 50 gals, water. 
To 100 lbs. dry stock add 90 gals, water. 

*' Boiled until a sample yields a jelly on removal : about 
one hour for wet and two hours for dry stock." The level 
is kept up by adding fresh hot water, well stirring. 

F. J. Machalski, Brooklyn, N.Y. (English patent, 5,821, 
1894), combines in one apparatus, which is necessarily very 
complicated, the several operations of extracting fat, boil- 
ing out the glue, bleaching, clarifying and evaporation. 
The raw materials are first subjected in closed receivers 
to the action of a solvent such as benzene, gasoline, or 
ether, aided by steam heat, to remove the fat. By an 
arrangement similar to the well-known Soxhlet apparatus, 
the solvent is recovered continuously, and the fat run off. 
Steam is then admitted to the receivers, and in condensing 
extracts the glue, the solution being forced into pans with 
steam coils, where it is bleached by sulphurous acid, clari- 
fied by alum and soda, and evaporated. 

Vacuum Pans and Multiple Effect Evaporators 

are much used in America. When, with the object of 
economy, large quantities of water are used for extraction of 
the stock, the liquors become so dilute as to require concen- 


tration. The boiling point of r liquor is higher than that of 
water in proportion to the percentage of BoHd matter, there- 
fore to boil down in the air would much injure the glne both 
in colour and tenacity. By exhausting the air by fans or 
pumps, the pressure is so reduced that the boiling point is 

greatly lowered, while the evaporation is accelerated. But 
complaints are made of the great waste owing to spray and 
froth being carried off in the steam. An American anthority 
states : " We|have found in the condenser water of vacuum 
pans from 01 to 0"25 per cent, of dry glue. For each gallon 


of liquor evaporated, twenty-five to thirty gallons of con- 
denser water are produced, therefore this means a very large 
VFaste of glue." 

" Swenson's patent " and the " Yaryan " are much need 
B.B multiple evaporators, besides the ordinary vacuum pans 
such as are employed for sugar boiling. 

Fis. 6. Yaryan Evaporator. 

Bone Glue : Acid Peocbss. 

In the boiling process the gelatine is dissolved out and 
the earthy phosphates left. In this method, the rationale 
of which is esplain«d in Chapter I., the mineral matter is 
removed by acids, and the skeleton of collagen left. 

The bones collected from miscellaneous sources and 
picked are coarsely ground to about three inches in diameter 
in a bone mill. 

The Croaskill Bone Mill (Fig. 7) is intended to be driven 
by a strap from the fly wheel of the common portable engine 
now 80 generally used for agricultural purposes. It consists 
of a pair of strong rollers made of wrought iron, with 


CABe-hardened cutters, and a revolving or oscillating riddle 
for separating the ground bonea as they fall from the cutters ; 
the whole carried by a substantial cast-iron frama They 
will grind from six to sixteeny^Ji©. per hour with a three 
to eight horse-power engine. 

Fig. 8 is a more powerful machine by the same makers 
(Crosskill) : when attached to a ten horse-power steam engine 
or water wheel it will crush and dress about fifteen to 

Fio. 7. CrosBkiU Bone Mill. 

twenty tons per day. It has two pairs of rollers with cutters, 
a revolving riddle for separating into rough, half-inch, and 
dust, and a friction sheave for preventing accidents to the 

The crushed bones are placed in vats containing cold 
hydrochloric (or in some cases sulphurous) acid of 7_B^ or 
1'05 specific gravity { = 10-6 per cent. HCl) for thick bonea, 
or half that strength for thin bones, and digested till they 
become soft and transparent. Sometimes a renewal of the 
acid is necessary. The phosphates can be precipitated from 
the acid solution by ammonia, or the whole evaporated with 


charcoal or silica and distilled to make phosphorus. The 
collagen is well washed with water till all acidity is removed, 
then placed in a series of closed iron digesters, each about 
eight or nine feet high, and holding eighteen j^filO, where 
they are treated with steam at two to two and a half atmos- 
pheres for three hours, the melted fats and solution of glne 

FiQ. 8. Crosskill Bone Mill. 

being ran off gradually into setthng tanks, and the rest of 
the operation conducted as usuaL 

Davidowsky states that 10 kilos of bones require 40 of 
acid, that they are covered for eight to fourteen days, then 
treated with fresh acid. 


SuLPHUEOus Acid Pbocbss. 

The familiar employment of sulphur dioxide in paper 
making, either as solution or as gas, suggested its use in the. 
manufacture of glue, especially that derived from bones. 

When ordinary bones are treated with a current of moist^ 
sulphurous acid gas, they absorb from 10 to 12 per cent, of 
their weight of the gas in the course of twelve hours. The 
amount may increase to 15 or 20 per cent, on longer treat- 
ment, but the excess will then disappear on exposure to air. 
Messrs. Grillo & Schroeder, of Dusseldorf, who patented 
this process (English patent, 2175, February, 1894), believe 
that this is simply due to the calcium phosphate present in 
the bones,, and remark that an absorption of 11 to 12 per 
cent, on the gross weight amounts to 16 or 17 per cent, of the 
inorganic constituents, and corresponds to the equation : — 

Ca3(P04)2 + SOa + HgO = 2CaHP04 + CaSOg, 

the sulphurous acid simply acting in the same way as sul- 
phuric acid does in the well-known manufacture of super- 
phosphate, but, being a milder acid than sulphuric, the 
alteration of the organic constituents which are available for 
glue stock can be almost entirely avoided. The acid phosphate 
is soluble in water, therefore the bones after treatment are 
easily disintegrated by boiling water, when a large portion 
of the lime remains in the sediment, while the gelatine is 

The process as commercially conducted is very similar to 
the well-known " siilphite '' method of treating paper pulp, 
and is carried out in iron cylinders, or better in close wooden 
vats lined with lead. 

The gas is usually generated in an impure form, with a 
large admixture of air and carbonic acid, by combustion of 
pjrrites and coal, 'of crude sulphur, or even of only a highly 
pyritous fuel. 


On the other hand, since it is well established that the 
absorption of a diluted gas is less ready, and is more wasteful 
than that of a gas in the pure state, the employment of 
a definite quantity of sulphur dioxide in a concentrated state, 
either prepared by the regulated burning of sulphur, or the 
decompesition-of sulphuric acid', yields more regular results, 
and a product of better quality. Liquid sulphur dioxide, 
which is now obtainable at a moderate price and in quantity, 
has the advantage that it yields a continuous current of pure 
gas of any required rapidity by simply opening a valve, and 
that the exact amount used can be ascertained by taring the 
containing vessel before and after the operation. 

The result of the process is a liquor which has been more 
or less bleached by the sulphurous acid, and contains a deposit 
of phosphate and sulphite of lim.e which require separating. 
Formerly it was the custom to allow it to deposit and run off 
the clear liquor, but the adoption of a filter press admits of 
using less water, and therefore saving the cost of evaporation 
and the injury to the glue by long heating. While insoluble 
the lime salts are separated in a concentrated state available 
for manure. It is necessary, however, for the sulphite, which 
may amount to 10 or 20 per cent., and is injurious to plants, 
to be oxidised to sulphate of lime, or gypsum, which is bene- 
ficial. The oxidation occurs with fair rapidity when exposed 
to the air with occasional turning over, but may be facilitated 
by the action of nitrous fumes or the careful addition of nitric 
acid, and of lime afterwards for neutralisation, when the 
** artificial guano " will contain the valuable constituents, 
phosphate and superphosphate, sulphate and nitrate of lime, 
and a nitrogenous residue from the gelatine. Another method 
of dealing with the. insoluble residue is to separate by mechani- 
cal means any unattacked bone (stated to amount sometimes 
to 10 per cent, of the original), to return this portion to the 
cylinders for further sulphiting, then to grind the rest to 


powder, and treat it with sulphuric acid as in the manu- 
facture of superphosphate, the evolved SO2 being used 

The liquor will be acid, and will require neutralisation 
with slaked lime. This is better done before the separation 
of the insoluble matter, as a further precipitate is occasioned. 
A small amount of calcium sulphite remains in solution in 
the glue liquor, and is useful by acting as a preservative. 
Glues for which sulphurous acid has been used, either in the 
preparation or for bleaching, will reveal themselves chemically 
by the presence of an unusual quantity of sulphate in the 

The necessity for using a fixed quantity of gas is illus- 
trated by the instructions in the above-mentioned patent. ' 
It is recommended to use half the number of cylinders or 
vats for new contact with the current of gas, while the 
other half, " after an absorption of from 10 to 12 per cent.," 
are resting to allow of the combination to take place, and 
are emptied and refilled in turn, the gas being passed 
in series through the active cylinders. When liquid sulphur 
dioxide is used, the weight of liquefied gas required will 
be 1 to 1*2 per cent, of that of the bones, and the vessel must 
be able to stand a pressure of two to three atmospheres 
(30 to 40 lb. per sq. in.). 

One advantage of the process is that the bones need not 
be freed from grease beforehand by solvents or otherwise, 
as the fat easily rises clean from the hot liquor, and can be 
recovered by skimming or by allowing to cool and solidify, 
and then removing. The treated bones are extracted three 
times with successive portions of water aided by steam for 
two to three hours each, the first and second extractions 
being usually mixed, the third used to extract fresh bones. 

It has been proposed to utilise the crude sulphite cellulose 
liquors from paper factories in the preparation of glue stock. 


For this purpose the Uquid contains at first too much lime 
in the form of calcium bisulphite ; this is removed by the 
addition of more lime in a manner similar to the well-known 
Clark's method of softening waters, when the calcium hy- 
drate combines with the calcium bisulphite to form neutral 
calcium sulphite, which is insoluble in water, and can be 
removed by deposition in vats or by a filter press, the 
equation being as follows : — 

CaKj (SOgJa + Ca (0H)2 = CaSOs + 2H2O. 

The calcium sulphite can then be made to yield sulphur 
dioxide gas by treatment in closed vessels with sulphuric acid. 
Or the SO2 can be liberated from the liquors themselves 
by the action of sulphuric or hydrochloric acid and steam, 
but the product is more impure than the first, and very 
watery^ (Grerman patent, 75391, 1893.) 

In patent 4356, 1883, several mechanical details of the 
acid process are described. 


After the extraction, the glue liquor is run into a large 
shallow vat, in which it is kept warm for a few hours, when 
the grease rises and is skimmed off, and the flocculent and 
fibrous impurities settle. The glue liquor can now be drawn 
off clear, if it has been properly made from sound stock, 
although the strength is not affected by a slight amount 
of turbidity. But the liquors from the third extraction are 
generally very cloudy, and often very dark; they are either 
at once made into **Eussian glue'' by incorporating with 
a white powder, or else must be clarified by subsidence or 
precipitation. Dilute liquors deposit their sediment more 
easily than strong ones. 

Alum, either alone or followed by a small quantity of milk 
of lime, has long been used for clarifying. The alkaline 
sulphate of the alum remains in the glue, which does not 


occur if crude sulphate of alumina is used : this salt is cheaper 
but is often very acid. The whole is heated to drive off the 
air bubbles, and cause the flocculent precipitate to coagulate : 
on standing for about four hours the liquid becomes clear. 
Albumen is sometimes used for the better qualities of glue, 
and generally for gelatine, but a cheaper substitute is fresh 
blood, which contains albumen and fibrin. Dry albumen is 
dissolved in cold water, or white of egg is used direct, if 
procurable. Before adding either of these substances the 
liquor is cooled to 54° C. (130° F.), and the clarifier well stirred 
in ; then the temperature is raised to about 93° C. (200° F.) 
when coagulation occurs, and the precipitate entangles the 
impurities and falls to the bottom, requiring, however, from 
twelve to twenty-four hours to clear. It is said that glues 
clarified with albumen have a characteristic " soapy '' smell 
and show a tendency to foam. 

Sulphited glues can be clarified by milk of lime, the 
calcium sulphite rapidly settling ; the liquor must be left 
slightly acid, or it does not clear. 

Cooling. — In some processes the concentrated glue 
liquor is cooled till set, and the block cut up into sheets 
by wires fixed in a frame. If the cooling is done slowly 
the liquefying bacteria will rapidly injure the glue, especi- 
ally in summer — the most dangerous temperature being 
about blood heat, or 37° C. (98° F.). Therefore, it is run 
into metal boxes, made of wood lined with zinc, or better, 
of stout sheet zinc or heavily galvanised iron. These hold 
about i cwt. each, and are of two shapes : one deep and 
nearly square, to allow of the settling of liquors containing 
solid impurities ; another long and shallow, for quick cooling 
of clear liquors. Iron should be kept from contact with 
the liquor, as it readily rusts, and causes discoloration. 

The cooling is effected by a current of cold water, 
where it is available, but often merely by cold air, aided 


by fans or blowers, in a room protected from heat or 

On a smaller scale for more expensive goods, the glue 
liquor is run on to sheets of glass, cooled by water, in 
layers about i inch thick. 

Formerly much of the cooling and drying was done in 
the open air, with great uncertainty and inconvenience. 
Large quantities of ice were also used. Several refrigerat- 
ing machines are now employed, which, by the evapora- 
tion of liquefied gases such as ammonia, sulphurous or 
carbonic acid, reduce a tank of brine to near freezing 
point. The temperature should not be allowed below 33° 
or 34° P., as if frozen the jelly is hard and difl&cult to cut. 
The brine circulates in iron pipes placed near the ceiling 
of the room ; they must be kept as clear as possible of ice 
and dirt, and the cooling house should be scrupulously clean 
and sweet. 

Cleansing Vessels, Etc, 

A series of two or three vats of hot water being pro- 
vided, the first one is used for removing the heavier coat- 
ings from the small coolers, nettings and ladles, and for 
dissolving the floor scrapings. When the liquor has be- 
come sufficiently strong, it is run into a boiler and used for 
common glue. The first vat is then filled from the second, 
which has been used for further washing the utensils. 
The third vat contains clean water for final cleansing; 
and may with advantage have an addition of zinc sulphate 
or carbolic acid as an antiseptic, the water being thrown 
away when dirty. 

Forming and Drying the Sheets. 

This part of the process has been considered as requir- 
ing the greatest care, and as being the most difl&cult and 


uncertain, whole batches being occasionally spoilt through 
formerly little-understood causes, most of which, however, 
have yielded to scientific investigation. 

From very early times, the process has been carried out 
by spreading the concentrated glutinous liquor, at a certain 
critical point judged by hand tests, as evenly as possible over 
hempen nets arranged in stages above one another, and 
dried by a current of warm air. It is clear that the drying 
of thick sheets in this manner is a slow process, and depends 
on the hygrometric state of the atmosphere. Souring and 
mouldiness are liable to appear, and the sheets are often 
contorted, uneven in thickness, and unequal in quality. 
In thundery weather it was considered almost impossible 
to obtain a good product. Glue liquor and jelly absorb 
ozone with avidity, and are decomposed by it into oxidised 
products with no gelatinising or adhesive power : this may 
be the reason why an approaching thunderstorm is reputed 
to cause great damage in destroying the coagulating powers 
of the glue soups, or causing the glue to turn on the nets, 
i,e.y to lose its consistency, and become liquid and foul. 
Crooke's edition of Wagner's Technology also speaks of 
the injurious effect of thunderstorms, without hazarding 
a reason. 

Cotton netting is now common in English factories. 
But the use of any weak or absorbent material is found 
to be attended with the following disadvantages : — 

1. Being freely handled in the making, it is almost 
•always impregnated with dangerous organisms which pene- 
trate the moist glue cake, and cause moulding or putrefac- 
tion. When this occurs, it is attributed usually to a state 
of the atmosphere, but if the cakes are examined, the 
alteration will generally be found to originate along the 
lines made by the netting. The fault could be cured by 
sterihsing the net for an hour at 100° to 120° C. in a hot 


oven, but besides the expense, the fibre is thereby weakened: 
Moreover, the spores of a few bacteria, such as Bacillus 
subtilis, which is widely distributed, and has the power of 
liquefying gelatine, will bear a heat of 120° C. for over 
*an hour, and still be capable of growing. 

2. However smooth the fibre, the glue will stick in places, 
leaving small remains, which being hygroscopic become 
*'sour," and set up the objectionable bacterial changes in 
the subsequent batches. 

3. Owing to sagging, rotting, souring, or wearing into 
holes, the b'fe of cotton or hemp netting is so short that the 
constant renewal is a considerable item. A whole batch is 
frequently spoilt by the fault of a net. In some works 
heaps of old netting are found, which become very putrid 
in the rain and sun, and give rise to mysterious bacterial 
inroads in the factory. In others they are regularly burnt 
under the boilers. |V^JivJ? ^t^JJ^/WyJu^ Ur^f^^ ^cJT 

4. The considerable overlap or selvedge required for secur- 
ing the edges of the nets involves a waste of the area, and 
also some difficulty in refixing. 

For these reasons metallic netting has been largely 
.adopted, and zinc is the metal chosen,. Ordinary " galvan- 
ised '' or zinc-coated iron was first used, but it was found 
that the zinc film rapidly wore away, and the exposed iron 
rusted and discoloured the sheets. Heaps of rusty galvanised 
netting accumulated as in the old cotton system, and were 
even more useless. Many patents were taken out for smooth 
non-adherent nets punched or cut in diamonds or squares 
and made of sheet zinc or various alloys, but beyond the 
expense, they were found to be deficient in strength and 
liable to corrosion and warping. The best material has 
proved to be a heavily galvanised iron-wire netting having 
not less than 15 to 25 per cent, of its weight of zinc. It can 
be strengthened by longitudinal and transverse wires or ribs. 



It must be examined by the microscope to see that it is 
perfectly free from holes or cracks, and should last at least 
two years in constant use. 

The temperature of drying rooms should not exceed 
21° C. (70° F.), or the glue will become fluid again and 
run through the nets, nor should the air be too dry in the 
first room or the cakes will become bent or cracked. 

In England, and on the Continent, the drying rooms are 
as a rule simply ventilated lofts, but it is quite possible to 
provide economically currents of air by fans screened from 
dust, and cooled in hot weather by the arrangement described 
in the section on cooling liquors, or warmed slightly in 
wdnter. The product is greatly improved, the process 
quickened, and uncertainty obviated. 

In America long drying galleries are constructed, some- 
times 250 ft. in length and 6 to 8 ft. square, with travelling 
platforms on rails carrying the sheets of glue on stout galvan- 
ised netting. Wood is found to be a better material for the 
galleries than stone or brick. The access of dust is a great 

Continuous Process. It was recognised that thinner 
sheets could be solidified and dried more rapidly and with 
greater evenness, and could then be amalgamated in layers if 
necessary into the thicker slabs which are customary. P. C. 
Hewitt, of New York, devised in 1894 (English patent, 
11,426) a machine for rapidly cooling and forming these 
compound sheets. (Pig. 9.) The hot glutinous liquid 
issuing by taps from the boilers is directed on to a species of 
table formed by six horizontal hollow cyhnders, close to- 
gether and revolving in the same direction, with a current 
of cold water circulating through them, so that the glue is 
rapidly cooled and solidified. A very slight film of grease 
prevents adhesion to the rollers ; the sheet at first requires 
guiding over them, but then passes continuously over an 



endless band of wire netting where it is dried by warm air. 
To make a twofold thickness the band is sloped downwards 
so as to deposit the sheet on a second sheet, carried on 
another wire band, and formed in the same way by a run 
from a lower tap ; a steam pipe inserted horizontally between 
the two sheets just before the junction causes partial melt- 
ing and therefore adhesion. A number of thicknesses can 
thus be joined, or a combination made of glues of diflferent 

Another form of Hewitt's machine consists of a horizontal 

V rr •-*--* -^ --* ,-» 

Fig. 9. Hewitt's Machine. 

cylinder cooled by water or refrigerated brine, dipping in the 
glue liquor, and carrying a thin coating of the latter, which 
rapidly congeals, is detached, cut by knives into the required 
lengths, and fed on to frames to dry. 

Kranseder & Leutsch, of Munich, patented drjring trays 
with longitudinal strips, on which the sheets of glue or gela- 
tine are laid. The trays are made thicker in cross section 
towards the exit end, so narrowing the spaces and in- 
creasing the speed of the air current, thereby causing the 
evaporation to be uniform. 



In the United States a large proportion of the glue is sent 
out in the crushed or powdered form, obtained by grinding 
in special mills. It is easier to use, but is subject to much 
dirt and adulteration. 

Brauer, of Brunswick (English patent, 2400, 1898), pre- 
pares a glue in a very porous condition, so that it can be 
easily powdered, by casting the hot glue from the boilers in 
small cakes, and then at once exposing to a vacuum ; when 
the glue becomes vesicular, it is allowed to cool and dry. 

• ^ •O 




■* % 

Fat Extraction, — We have descnbed how the fat is 
rteSioved by boiling steaming, draiftifa^T4>ressure, or«si>Ivents, 
and also many patents for the purpose. While it is necessary 
for some applications that the glue should be entirely free 
from grease, the density of bones and their gelatinous basis 
render complete removal very difficult. Boiling with water 
and skimming give very imperfect results, as much of the fat 
remains emulsified. Extraction of the original bones in the 
ordinary manner with benzene, bisulphide of carbon, ether, or 
light petroleum is equally incomplete. In Seltsam's process 
the solvent is boiled with the bones (previously coarsely 
crushed and the dust sifted out) in a strong closed vessel, so 
as to obtain a higher temperature, greater penetration, and 
avoidance of loss. The vapour ascending condenses in the 
pores, extracts the fat, and collects under the grid at the 
bottom as a layer of solution which is subsequently dis- 

Leuner's apparatus works on the Soxhlet principle, 
without pressure (Fig. 10), using solvent and steam simul- 
taneously. The crushed bones are placed in J above the 
perforated false bottom, M. C is a steam pipe, by means of 


which the bones are steamed as a prelimiafuy, the surplus 
steam escaping through the exit pipe, Q. After steaming, 
water £ind benzene are run in from the reservoir, L, into the 
space under the false bottom, and heated up by the steam 
coil, 0. The evolved vapours are condensed in the worm. A, 
and at first run back over the bones through the cock, F, the 
vapour passing upwards to the worm through D, and the 
condensed liquid being divided into separate streams by the 

Fia. 10. Leuner'G Apparatus. 

Spreading plate, G. After some time the cock, H, is opened, 
flo that the condensed liqnid runs into the reservoir, L, instead 
of flovring back into J. When all the solvent has been vola- 
tilised nothing but water coodensea in the worm, which is 
known by means of a sampling cock attached to J ; the 
draw-off cock, Q, is then opened, and the watery gelatine 
solution and oily matter run off into a suitable separating 
receptacle ; J is then discharged through a manhole and 
refilled, and the whole operation repeated. 


Bone fat, or bone grease, differs much in character accord- 
ing to the preparation. From fresh clean materials, as 
obtained in the manufacture of gelatine, it is white or 
yellowish, soft, and has little odour or taste. Its melting 
point is given by Schaedler as low as 21*2° C, so that it* is 
liquid in summer, hence it is often called *'bone oil," a term 
also applied to the product of destructive distillation (see 
below). Lewkowitsch describes it as '* not readily turning 
rancid, for that reason a valuable lubricant ". But it is 
chiefly used for mixing with neat's foot and other oils. 

Most of the crude bone grease of commerce is, however, 
dark coloured, unpleasant in odour, of much higher melting 
point (bone grease 44°, tallow 46° to 50° C), and contains 
free fatty acids. It is used for coarse lubricants, or bleached 
for candle making. It generally retains phosphate of lime 
and other calcium salts, which must be extracted before it can 
be used for soap making. It is therefore placed in a lead- 
lined tank with a suflScient quantity of dilute sulphuric or 
hydrochloric acid, and steam blown through to agitate it 
thoroughly. Sulphuric acid is cheaper and acts less on the 
lead, but forms an insoluble deposit of sulphate of lime. 

The residue, after extraction of glue and fat, still contains 
about 5 per cent, of organic matter. It is treated with 
sulphuric acid for the manufacture of superphosphate or 
used directly in compounding artificial manures. 

Spoilt bones and waste unfitted for glue are submitted to 
destructive distillation in iron retorts, yielding (1) inflammable 
gases, (2) bone — or DippeFs — oil, a dark-brown fetid mixture 
of ammonium salts and a number of organic compounds, 
including phenol, aniline and pyridines (on redistillation a 
tarry mass is left which is employed in making Brunswick 
black), (3) a final residue of animal charcoal. 



In this connection the qualities most in demand are : — 

1. First and foremost tenacity, or the power of resist- 
ing rupture. This is ascertained by the methods given in 
the section on Testing, Chapter V. 

2. Elasticity, or absence of brittleness, and power of 
yielding or stretching slightly without fracture. A ' glue 
which is too hard, or made with too Uttle water, may re- 
sist an enormous strain if applied steadily, and yet will 
break with a lower force coming with a sudden jerk. A 
sample that is somewhat elastic is preferable in all cases 
except where very hard wood is used with the view of great 
rigidity : even in this case the end is better attained by 
clamping or by a dovetail joint. In applying the weight 
in the testing machine, it will be noticed by the movement 
of the lever that an elastic glue gives way slowly before 
it breaks, whereas a harder and more brittle one resists up 
to the point of sudden fracture. Therefore, a good idea 
may be formed in this way of the elasticity. A more exact 
determination may be made by the apparatus used for 
testing this quahty in iron and steel bars, but the operation 
is rarely required. 

The strength of glue, if weU used, is generally greater 

than the wood which it unites, so that the fracture takes 

place through the wood and not through the glue joint. 
5 (65) 


3. Covering Power. — This is a very important matter, as 
it controls the economy in use. It is ascertained by con- 
sidering three factors : the water-absorption, the tenacity of 
the jelly, and the viscosity of a solution of known strength. 
The verdict of these numbers is usually in the same direction ; 
in the few cases, where they differ widely in their indications, 
it may be suspected either that the testing has been at fault, 
or that, as has often been found, the glue has possessed some 
peculiarity which in practical use has made it of less value. 
At any rate, if the three factors are ascertained, a sure judg- 
ment can be formed of the value. It would be well if buyers 
were to inquire for, and make themselves acquainted with the 
meaning of, these constants, and be able, as far as possible, 
to verify them in the workshops, instead of trusting to trade 
descriptions, or to prices, which, as we have explained else- 
where, are often artificial. 

4. Keeping Qualities, — Although this is here less indis- 
pensable than in some other uses of glue, considering that 
the right quantities are better made up when wanted, it is 
inconvenient when a made glue that has to be kept turns 
sour or mouldy directly, or the same thing happens to joints 
in damp weather. Some liquefying bacteria, or their spores, 
such as Bacillus subtilis, are not easily destroyed by heat, and, 
if present in the interior, are quite capable of rotting a recent 
joint away : when it is dry, and properly set, they can hardly 
enter from the outside. 

Colour, except for fine cabinet and inlaid work, is of less 
importance. Dark glues are generally preferred by carpenters, 
as stronger in proportion to price. A lighter article has either 
been made of selected material, therefore it is expensive, or 
it has been bleached, and has probably been reduced in 

The ordinary glue pot is sufficiently famiUar. It is prac- 
tically a water bath, consisting of an outer iron vessel of 


water, into which fits the inner receptacle for the glue. The 
adoption of this, the earliest form of water bath, and still the 
only one universally known, is due to the knowledge that 
glue must not be heated over an open fire, or it would burn, 
and that undue heating damaged its setting and adhesive 
properties. For the latter reason there is doubtful utility in 
the practice sometimes recommended of adding salt or 
chloride of calcium to the water to raise its boiling point, 
under the plea that the glue must be used hot. I have 
found that a temperature of 70° or 80° C. (156° to 178° R), 
which is easily attained by a bath of water alone, is the least 
injurious to the chemical and physical properties of the glue, 
while it is hot enough for any work when quickly done. The 
habit of '* boiling well " and of using '' blazing hot " has led 
to much bad and brittle work. The salt or chloride of 
calcium mentioned above rapidly corrode the iron. Even 
water must not be left standing in the outer vessel, and if 
the use be intermittent the inner receptacle should be boiled 
out in a saucepan, and the whole cleaned and dried for the 
reception of new glue in the quantity required, and that 

The best method of preparing glue for use, where there is 
time, is to first break it with a hammer, then to beat it in a 
large iron mortar, covered with a cloth or a piece of cardboard 
with a hole in the centre for the pestle, until the largest pieces 
are about the size of peas. Any trouble spent at this stage 
will save time in the next. Cover it with five to ten times its 
weight of cold water, according to the consistency required, 
and let it stand, covered from dust, in a cool place for some 
hours, or even overnight. By this treatment, if it is good 
glue, it will become soft and much swollen, with clear water 
above : if liquefied, with water foul and turbid, it is bad 
and should be rejected (Chapter V.). The water being poured 
off, any hard lumps are picked out and soaked in water a 


little longer. The soft mass is transferred to the glue pot 
and melted without adding more water. It is usually kept 
near the boil for about iifteen minutes before using. Any 
remainder becomes on coohng a very stiff jelly, which can be 
just warmed up two or three times without injury. A wire 
across the inner vessel enables the brush to be cleared. 
It ia better, wherever possible, to heat the wood, short 
of -warping, than to overheat the glue. The layer should 

be spread steadily and evenly, seeing that there are no air 
bubbles : the pieces of wood are then pressed together, and, 
if possible, tied or clamped. The excess of glue may be 
carefully scraped or sponged off, but except in corners it is 
better left on till cold, as it makes a firmer and more air-tight 
joint, and enables one to judge when the glue is sufficiently 
set, which takes one to three days, according to the season. 
Within limits, the closer the wood surfaces are brought in 
contact the more permanent the adhesion. The matter mnst 


of course be learnt by practice ; we merely wish to indicate 
the principles. The adhesion to resinous woods, as yellow 
pine, is promoted by first chalking the joints. 

Where large quantities of glue are required, the outer 
jacket is heated by steam. " Eichard*s Steam Glue Oven '* 
is an apparatus in use in many workshops, and is represented 
in the figure. 

In No. 1 size the central pot, used for melting, holds 1 
gallon, the end ones J gallon each. In No. 2 size the 
pots are all 2 gallons. The. floor space occupied is, in 
No. 1, 3 X 2 ft. ; in No. 2, 3 x 3 ft. 

Copper glue pots are quicker in heating, but are liable to 
turn green and be corroded by the slightest acid in the glue. 
Well tinned iron is preferred in most workshops. China is 
slower in heating but is cleaner ; sometimes, however, when 
left to cool, the contraction of the glue will actually chip 
away portions of the material. 


The foundation wood is smoothed, then finely and regu- 
larly roughened by a toothing plane or machine, or even a file 
or rasp for small work. Size (1 part good Scotch glue with 
50 of water) is applied hot, and any defects made good by 
stopping with a thin paste of fine plaster of Paris and hot 
glue. When dry, the surface is again levelled. The glue is 
then spread on the foundation wood quickly, but with great 
care as to evenness. For large work revolving brushes are 
used. At this point the fault of frothing, or " foaming," 
which attends some otherwise good glues (Chapter V.) 
becomes prominent ; it almost certainly depends on the 
presence of viscous bodies such as mucin or the slimy sub- 
stances into which glue is converted by long heating, as 
described in the first chapter (p. 9). It would be interest- 
ing to examine whether samples which show a divergence 


between the indications of the shot (jelly) test, the absorption 
of water, and the viscosity, show also a difference in the 
tendency to froth. 

The fault of foaming has occasioned considerable trouble, 
and at once condemns a glue for certain purposes, whatever 
grade be claimed for it, and quite independently of its cost. 
In America, where application of glue by machinery on the 
large scale is commonly practised, it is said that '* while 
a large number of glues made in the States have this draw- 
back, none of those coming from Europe are free from it ". 

The causes assigned in the trade are : — 

1. Prolonged cooking. 

2. The presence of hydrate of lime through careless or 
insufficient washing after liming. 

3. Use of acids in cooking, bleaching or clarifying. 

4. Zinc salts, or even zinc white as used for " coloured 
glues '' (p. 20). 

■ Before the glue chills the veneer is laid on and caused to 
adhere either in the dry way, by pressing with a heated piece 
of wood called a ** caul '* cut to the shape of the surface, or 
in the wet way, by damping with hot water while the glue is 
still hquid, and tapping and rubbing with a kind of mallet 
called a ** veneering hammer,*' and with hot irons, till the 
excess of glue is squeezed out at the edges. The work is 
now weighted till dry, again sized, smoothed and polished. 

Papek Making, etc. 

To make the surface less porous, paper is either " engine " 
or " tub " sized. The former is effected with a resin soap, the 
latter by passing the sheets through a warm solution of 
gelatine or of light-coloured glue, which must not be acid, 
and should be nearly free from smell. Paper makers are 
said to have a strong preference for German Cologne glue 
and its imitations. 


Animal Size for Paper Making 

must be drawn off with the least possible amount of heat. 
According to Clapperton, when three or four extractions are 
made it is a good plan to run off the first from one pan at the 
same time as the last from another, so that by mixing the two 
the strength may be kept uniform. 

Soap and alum are also added, the former to improve the 
finish and feel of the paper, the latter to make the size keep 
better as well as to make it penetrate more easily, besides 
giving more solidity and hardness. It has, however, an 
action on certain colours. 

Paper has frequently an unpleasant odour derived from 
the size, which, even when made from the best grades of 
hides, is not entirely free from smell, but when low grades 
are used, mixed perhaps with cheap glues, the odour is often 
offensive. ' 

Prepared Casein is also largely used for sizing. A 
company is being formed in America for manufacturing 
it on a larger scale than hitherto for this and other pur- 

For wall papers the glue need not be of high grade, but 
should be of good colour, and especially free from grease (see 
(Chapter V., Grease Test). Paper-box makers require a low 
grade glue, of fair colour, drying slowly — because of warping 
— and free from strong odour. 

Straw hat makers usually employ the best grade of hide 
glue, of light colour and inodorous : it must not be affected 
by oxalic acid, so that lime must be almost absent. 


For the better classes of work the glue should be naturally 
pale and strong and without marked odour. Some inferior 
glues which have been chemically bleached turn almost black 


in the pot, owing to the bleaching agent not having been 
properly removed or neutralised. They are generally classed 
as " French," although this may be considered as a trade 
name rather than as referring to the country of origin. 
Besides the water and other tests (see Chapter V.) the use 
of litmus paper should never be neglected. Any glue which, 
after being soaked in distilled water, sensibly reddens blue 
litmus, is unfit for better work, as it will injure many delicate 
colours. It has been proposed to use chondrin glue, as 
chondrin, not being precipitated by corrosive sublimate 
(mercuric chloride), one per 1,000 of this antiseptic can be 
incorporated with the glue, to keep away the parasites which 
cause so much damage ; volatile compounds, like camphor 
and essential oils, may also be added, but the odour is open 
to objection. Mercuric chloride is one of the most powerful 
of disinfectants ; it is inodorous, but is very poisonous, and 
is apt to cause discoloration, either by reduction, by the 
action of sulphuretted hydrogen from the air or gas, or by 
acting on sensitive pigments. It precipitates ordinary glue, 
therefore, cannot be used with it^^ ^j^ (^^^^ l^\/^'J^Jy'i ■ 

It is recommended that, in preparing glife, a few cakes 
should be broken in pieces and placed in water for twelve 
hours, then boiled and turned out into a pan to get cold ; 
when cold pieces may be cut out, and placed in the glue pot 
as wanted. It should be kept protected from^ies and dust. 
Glue should always be used as hot as possible, but not kept 
hot longer than is necessary, as it loses strength in the wAy t^ Ir 

we have described (Chapter l-).^'^^^^^,^^^'^'''^ ^ 

Zaensdorf {Art of Bookbinding, p. 93) recommends that 
while '' paste should always be used for Morocco, calf, Eussia 
and vellum, all leather with an artificial grain should be glued ; 
the turning-in may be with paste. The glue gives more body 
to the leather, and thus preserves the grain." The old binders 
of white vellum books gave the boards a thin and even coat 


of glue, which was allowed to dry before putting on the 
covering. Roan should be covered with glue, and turned in 
with paste. Cloth is covered by gluing all over and turning 
in at once : gluing one cover at a time, and finishing the 
cover of each book before touching the next. Velvet should 
be covered with very clean glue not too thick, first gluing the 
back, and letting that set before the sides are put down. The 
sides should next be glued, then the velvet laid down and 
turned in with glue. Any finger marks may be raised by 
holding the book over steam, and carefully using a soft brush. 
Silk and satin are first lined with thin paper cut to size, and 
fastened with thin clean glue. When dry, the book is covered 
in the same way as with velvet. 

The same authority warns us that calf is liable to be 
stained black by glue unless certain precautions are taken 
which are described (p. 88) : all glue must be removed from 
the back and sides before covering with calf. Morocco may 
have the back glued, as it will not show through, and will 
faciUtate the adhesion of the leather. 

In fastening together the sections of a book at the back, a 
commoner grade of glue is often used : it is to be applied with 
a brush, and not as in some shops with a handful of shavings, 
which is a very clumsy and wasteful practice. In Germany 
the glue is rubbed in with a special hammer, and the surplus 
taken ofif with a brush. The drying should be effected spon- 
taneously in a press in a dry room free from dust, and never 
by artificial heat. 

Size for finishing and gilding was formerly made from 
vellum scraps, but can now be bought ready for use, or made 
from a thin soluj;ion of a pale glue. If too strong, it will 
make the work brittle : some papers require a stronger size 
than others. The following receipts for bookbinders' size 
are given : — 

No. 1. Water, 1 quart ; powdered alum, ^ oz. ; Eussian 



isinglass, 1 oz. ; curd soap, 20 grains. Simmer one hour, 
strain through linen or a fine sieve, and use while warm. 

No. 2. Water, 1 gallon ; best glue, i lb. ; alum, 2 oz. 
Prepare and use as above. 

No. 3. Water, 1 quart ; isinglass, 2i oz. ; alum, 120 

Compositions for Printing Rollers 

all contain gelatine or glue. The following receipts are 
used : — 










Glue . 








Treacle . 


• « ■ 







Paris white . 


• « • 

• • • 

• • • 

• « • 

• • • 

• ■ ■ 


Sugar . 

• « • 


• • • 

• « • 

• • ■ 

• • • 

• • • 

• • • 


• ■ • 


• • • 

■ • • 


• • • 

• • • 

• • • 


■ ■ • 


■ • • 

« ■ • 

• ■ • 

• ■ • 

■ • • 

• « • 

India-rubber in 

naphtha . 

• ■ • 

• • • 

• • • 

■ « • 


• • • 

• • • 

• • • 

A patent roller composition is thus made: 32 lbs. of 
gelatine and 4 lbs. glue are softened in cold water and 
melted in a glue boiler. To this is added 4 lbs, glucose, 
72 lbs. glycerine and 1 oz. methylated spirit. The whole is 
then digested for four to six hours and cast into rollers. 
This composition is claimed to be unaffected by temperature, 
to retain its elasticity and not to shrink. 

In practice, it is found that all these compositions from 
the cleansing and remelting become gradually sticky and 

To partially overcome this difficulty, formaldehyde is 
added to the roller composition, which renders the gelatine 
insoluble in water, and so prolongs the life of the roller. 

Very similar compositions are used for the beds of 
hectographs. Chrome glue will be spoken of under 
Gelatine in Chapter IV. 

uses of glue. 75 

Glue in the Match Manufactuke. 

A very large quantity of glue is used for this purpose^ 
especially in England, where it is about the only binding 
material employed. Very much depends on the quality of 
the glue and its drying properties ; even the colour is impor- 
tant as affecting the bright appearance of the article. The 
** dipping composition" for matches containing vitreous or 
ordinary phosphorus is a bath of glue of 25 to 50 per cent, 
strength to which the requisite amount of an oxidising 
agent, like potassium nitrate or chlorate has been added, 
kept at a temperature of 38° C. The phosphorus is 
cautiously put in ; it melts, and is stirred to an emulsion, 
when the sand, glass, or other friction agents are incorpor- 
ated. The object of the glue is to protect from oxidation^ 
without diminishing the sensitiveness. Match factories in 
the United States are said to prefer ** Irish glues" as they 
**mix easily with the phosphorus, and stick well to the 
wood *'. 

Glue is also used as the binding material in the heads 
and rubbers of safety matches. 

Sand, glass, and emery papers and cloths are made by 
coating the surface with a thin uniform layer of strong glue, 
and sifting the powder evenly over. Very common or over-, 
boiled glues have a sale for fastening the bristles of cheap 
brushes for toys and for sticky fly-papers. 

GtUE AND Gelatine in Substitutes fob Othee 


By appropriate means and mixtures, glue can be rendered 
insoluble and either soft, pliable, and elastic, or so hard as to 
be capable of being worked with a lathe and polished. It is- 
not surprising, therefore, that glue and gelatine have been 
made the basis of a multitude of inventions for imitating 


leather, caoutchouc, woods, or even stone, to be moulded, 
stamped, or carved, for objects of art or even of utility. 
Cheapness and facility of working are the main conditions, 
since none of these preparations can be equal in durabiUty 
or strength to the natural products, while many of their 
physical properties are essentially different. 

Ive's patent. No. 28,817, 1898, mixes gelatine, glycerine, 
potassium bichromate (to cause insolubility) and ground 
cork, and moulds by heating to the required shape. " Elastic, 
tough and insoluble." 

The sulphonic acids obtained by the action of fuming 
sulphuric acid on petroleum, mineral wax, or rosin oil have 
the property of precipitating glue or gelatine from slightly 
acid solutions, forming an elastic caoutchouc-like mass, 
capable of being rolled or drawn into threads. (English 
patent, 19,502, 1890.) 

The addition of ichthyol oil, obtained by the distillation 
of bituminous shales containing fossil fish-remains, to a mix- 
ture of gelatine and glycerine is said to give a compound 
** closely resembling rubber, and even more elastic ". 
{English patent, 7,745, 1898.) 

Leather and Art Work. 

Leather waste and cuttings accumulate in great quanti- 
ties. They can be utilised in several ways. 

(1) Freed with some difficulty from tannin, etc., they are 
converted into glue as seen in Chapter II. 

(2) Taking a considerable time in rotting, they furnish 
a slow, very lasting manure, but not free from danger, owing 
to the chemicals sometimes present. 

(3) For making cyanides. 

(4) For consolidating^rxificially. 

Eapeaud (English patent, 8,221, 1888) ground them to 
a fibrous paste in the following proportions: — 


Leather refuse, 78 to 90 ; gelatine, 4 to 8 ; tallow, 1 to 2 ; 
glycerine, 5 to 12 ; forming into plates which were dried and 

calendered, ♦ftvr OjJ><^*7^ )cjd!uh^ 

Boult (English patent, 15,404, i897) grinds leather paste 
and glue with hemp, flax or jute to give a fibrous structure. 

Other patents for artificial leather are : Thiele and Stocker 
(English patent, 8,960, 1895). 

An emulsion of 5 parts of bone glue in hot linseed oil 
or glycerin and paraffin is well mixed by agitating in a closed 
vessel with a solution consisting of 5 parts of caoutchouc 
and 10 parts of resin dissolved in 100 parts of carbon bisul- 
phide and a little spirit. This mixture is then worked up in 
a rag engine with 50 parts of finely divided cotton or wool 
fibre, and 50 parts of ground ivory, cocoa or para nuts, 
previously boiled in glue solution and dried. Five parts of 
carbonate of lime, and, if required, colouring matter are added 
to the mass in the rag engine, and it is then air-dried for some 
hours, and rolled to the necessary thickness. The product is 
then treated with a mixture of concentrated nitric and sul- 
phuric acids, washed, dried, and again passed between rollers 
heated to 50° C. 

Schmiedel, Austria (English patent, 8,847, 1895). 

For bookbinding, or other fancy work, cotton or Unen 
fibre is glued on to stout paper, calendered and lacquered, 
coloured or embossed. 

Billing and Latelle (English patent, 22,965, 1896). 

Cotton wool or other fibrous material is treated with a 
mixture of gelatin and methylated spirits or naphtha, and 
rendered pliable by the addition of treacle -or glycerin, and 
after being dried it is subjected to a bath of formalin, alum, 
or tannin to harden the gelatin. It is then washed, dried 
and embossed, etc., in the usual manner. 

Conn, Mudon and U^ll (English patent, 30,357, 1897). 

Gelatine mixed with enamel pulp, glycerine, spirit, shellac, 


formalin and colouring matter. Prepared in thin sheets and 
backed with some fabric. 

Moerch, Denmark (English patent, 4,235, 1899). 

Vegetable fibres and wool, saturated with a compound of 
linseed oil, resin, turpentine, glue, casein and wax, with a 
small quantity of borax and bichromate of potash, soaked in 
acetate of alumina, dried and pressed between heated rollers : 
the product is said to closely resemble sole leather and to be 
worked like it. 



The chemical reactions of gelatin may be here noticed ; 
most of them also apply to glue solutions, and some have 
been already given in Chapter I. 

In warm solutions, or in those suflBciently weak to be 
not gelatinous, acids, alkalies,^ and most mineral salts (such 
as those of Al, Fe, Cu, and Pb) occasion no precipitate. 
Chondrin gives precipitates with most of these. 

Phosphomolybdic or phosphotungstic acids cause volu- 
minous precipitates, the complete separation requiring twelve 
to twenty-four hours : the nitrogen can be determined in 
the precipitate, but includes other substances besides gela- 

If the solution be evaporated to near dryness, and then 
treated with a saturated solution of ammonium, magnesium 
or zinc sulphate, the gelatine is thrown down in stringy 
masses, along with the albumose, but not the peptones : 
the latter are almost absent in good gelatine; this process 
is therefore valuable quantitatively. 

On adding a saturated aqueous solution of picric acid, 
a precipitate is produced which dissolves on shaking, but 
is rendered permanent by further addition of the reagent. 
On heating, the precipitate dissolves, reappearing as the 
solution cools ; on shaking, a yellow sticky mass is formed, 

^ Bone glues may give a turbidity with alkalies, owing to phosphate of 




leaving the liquid nearly clear. (Nearly all proteids, except 
mucin, give at once a permanent precipitate.) This be- 
haviour is peculiar, and may be occasionally useful for the 
recognition of gelatin (Allen and Tankard). 

Platinic chloride, mercuric chloride in excess, and basic 
lead acetate, throw down gelatine more or less com- 
pletely.. Acetic acid and potassium ferro-cyanide give no 

Copper sulphate and soda give a violet-blue colour, the 
so-called biuret reaction. 

An acid solution of chromic acid is said to precipitate 
gelatin, but not peptones. 

Chlorine water in excess produces a turbidity with very 
small quantities of gelatine. If a current of chlorine be 
passed, in excess, through a solution containing not more 
than 0*2 to 0*5 per cent, of gelatin, a very insoluble floccu- 
lent precipitate is formed, which, after filtration and wash- 
ing, may be dried in vacuo and weighed ; or the nitrogen 
may be determined in the washed compound without 
drying. This process throws down all albuminoids, but 
not bases such as creatine. The weight of the chlorine 
precipitate, multiplied by 0*74, gives the amount of dry 
and ash-free gelatin in the solution. If the nitrogen be 
determined, multiplying by 6*5 will give the weight of 
gelatin present. For further details see a paper by S. 
Kideal and C. G. Stewart in the Analyst for September, 
1897, also on the use of bromine for the same purpose, 
by Allen and Searle, in the October number. 

By artificial digestion, or the action of acids or of bac- 
teria, gelatin passes into gelatoses, similar to the albumoses 
from albumin, and gelatones, or gelatine-peptones. The 
latter do not form a jelly. For Klug*s investigation of 
these substances see Centr, Physiol., iv., 189 ; Ghem. Soc- 
Abstracts, Feb., 1891, p. 232. He obtained in addition 6*7 


per cent, of an insoluble residue, apoglutin, and gives the 
elementary composition of the three bodies as: — 

C. H. N. O andS. Ash. 

Glue . . 42-76 700 15-61 3464 0-88 

Apoglutin 48-39 7*50 1402 3009 622 

Glutose (gelatose) 40*06 702 1586 3706 214 

Physical Properties of Gelatin Solutions (Bayley, 

Brit, J, Phot, xlv., 764). 

Bayley finds that the setting point of a solution of gelatin 
is about 8° C. lower than its melting point ; and the rise in 
melting and setting points according to the percentage of 
gelatin is more marked in dilute than strong solutions.. It is 
well known that by prolonged heating above its melting point 
of a gelatin solution its setting power is gradually diminished 
until it remains hquid at ordinary temperatures, but the 
melting point of good gelatin is at first slightly raised by 
boiling. Bayley states that the time of boiling- may be 
approximately estimated by the change of colour. About 
150 cc. of formalin per litre prevents the remelting of a 
5 per cent, solution of gelatin. The addition of alums con- 
siderably lowers the melting point of a solution of gelatin. 

Preparation of Liquid Oelatin (Mills* patent, 8,847, 1895). 

Gelatin is permanently liquefied by digesting at 100° C. 
with five times its weight of water, and one-twentieth of its 
weight of slaked hme for two hours. Other fixed alkali may 
be used instead of lime, and the proportion is varied accord- 
ing to the quality of the gelatin. 

In this patent, use is made of liquefying bacteria, such as 
B. liquefaciens for permanently liquefying the gelatin. A 
10 per cent, solution of gelatin, containing a small quantity of 
meat extract, and a trace of carbonate of soda to hasten the 
growth, is inoculated with the liquefying organism and incu- 
bated at about 25° C. for a few days. 

The products from either process are claimed to be of 



value as " gums, thickeners, detergents, emiilsifiers, and 
media for photo-lithographic and other photographic pur- 
poses *\ 

Gelatine in Photography. 

In gelatine for photographic purposes, purity, neutrality, 
absence of colour, and indifference to the silver and other 
salts used, are the chief requirements. The presence of 
chondrin, according to Colonel Abney, is very objectionable : 
the chrome - alum test for this impurity is given under 
Chondrin in Chapter I. M. L. Soret (Archives des Sciences 
Phys. et Nat, x., 139 ; Comptes Rendus, xcvii., 642) and W. 
N. Hartley (Chem, Soc, Trans., 1887, 58) have investigated 
the optical properties, particularly the transparency to actinic 
rays in and beyond the violet region, which is of very great 
importance. The latter observer states that " the examina- 
tion of specimens of gelatine shows that the difficulty of 
obtaining photographic plates sensitive to the most refran- 
gible rays Ues entirely with the character of the gelatine. 
Ordinarily the spectrum extends to wave-length about 2,146 
Cd. Some plates prepared many years ago by Wratten & 
Wainwright, called ordinary dry plates, were used for 
photographing a series of metallic spectra which extend to 
wave-length 2,024 Zr, and there was little doubt that they 
were capable of receiving impressions of lines still more 
refrangible. Since then plates of every kind by every maker 
have been tried, but most of them transmit nothing beyond 
2,146 Cd. A sample from Mawson & Swan was recently 
found to photograph as far as 2,024, but half a dozen other 
batches from the same makers were deficient in this respect, 
although supposed to be of exactly the same character. As 
the plates were prepared in precisely the same way there can 
be no doubt that the difference was in the gelatine, which 
must have contained some very slight trace of impurity 
which could not otherwise be detected." The method of 


experimenting adopted was to allow the liquid to dry on 
plates of quartz (which are almost completely diactinic, or 
permeable to the rays), and then to photograph the spectrum 
transmitted by the films. Hartley also examined : — 

'* (1) A yellow specimen sold for photographic purposes, 
said to be of Nelson's manufacture, but without any distinc- 
tive label. A solution, containing 5 per cent, of the solid and 
1 millimetre in thickness, was allowed to dry on a quartz 
plate. It transmitted a continuous spectrum to wave-length 
2,265, but beyond 2,313 the rays were weak. 

'* (2) A very fine colourless sample made in sheets, which 
in the original form transmitted all rays to 2,265.'* Dried as 
above on quartz, it transmitted all rays to 2,265 in full 
intensity. He finds that gelatine diflFers from albumin, 
casein, and serin ; while the former is highly diactinic, and 
shows no absorption bands, the latter give absorption bands 
as follows : white of egg, 2,880 to 2,650 ; pure albumin, 
casein and serin, 2,948 to 2,572. 

Gelatine plates in photography were first used by W. H. 
Harrison in 1868, while experimenting on the lines of Sayer 
and Bolton, using gelatine instead of collodion, but with 
only limited success. Dr. Maddox, and afterwards Dr. 
Burgess, made and sold a gelatine emulsion which was the 
first practical formula used. 

In 1878 Kennett found that by keeping the emulsion 
before coating the plates at 90° F. for some days, the speed 
was so increased as to admit of instantaneous exposures ; 
subsequently the process was shortened by boiling the emul- 
sion with part of the gelatine and adding the remainder. 
Monkhoven, in 1879, found that ammonia also increased 
the speed of the emulsion. 

The incorporation of small quantities of certain dyes, 
such as eosin and some oranges and yellows with the 
gelatine film, aflFects the sensitiveness of included silver salts 


to diflferent rays of the spectrum. This is called •* ortho- or 
isochromatic '* printing. 

In EngUsh patent 11,821, 1895, a layer of resin is applied 
to assist the stripping of the gelatine coatings from paper 
supports in the manufacture of photographic films. 

Effects of Lead Nitrate and Ammonium Sulphocyanide. 

A solution of lead nitrate at the ordinary temperature 
dissolves gelatine; a concentrated solution at 25° C. dis- 
solves 24 per cent. 

In ammonium sulphocyanide it is still more soluble ; 5 
grammes of the salt in 10 cc. of water will dissolve 5 grammes 
of gelatine ; 100 cc. of water with 1 gramme ammonium 
sulphocyanide at 23° C. dissolves 2^ grammes of gelatine. 

This salt, therefore, which is much used in fixing baths 
for gelatine plates should not be made up too strong. 

Silver bromide is appreciably soluble in gelatine solution, 
giving an opalescent liquid, reddish by transmitted light, 
and forming on glass an almost perfectly transparent layer. 


includes such inferior gelatines as are used, not for ad- 
hesiveness but for **body," in filling porous surfaces such 
as wood or plaster, stiffening and weighting textile fabrics 
in paper manufacture, and as a foundation for oil paints or 
varnishes. The name applies to the kind that is sent out as 
a jelly, or tub size, and to that sold in dry cakes, which are 
usually thinner and less coloured than those of glue. Even 
the best sheet gelatine is used for some fine purposes. Many 
sizes contain much chondrin, which for this purpose is not 
considered to be disadvantageous. 

An outline of a simple process used in an English factory 
devoted to making tub size may be interesting. 

The material is obtained from the tan-yard ready limed 
and freed from hair, and consists mainly of " faces " of 



bullocks or cows (the noses being cut off as food for dogs). 
It is soaked again in weak lime water and re-washed, then 
placed in ** coppers ** made of wrought iron welded in one 
piece, and holding about 10 to 20 gallons, enclosed in outer 
jackets of the same material containing water which is 
kept well boiling. There are six coppers, about five feet high 
by three feet in diameter. The charge of each is about 
i cwt. ; it is covered with soft water and well stirred with 
sticks for two hours, the scum and dross being occasionally 
skimmed off and thrown away as useless. At the end the 
size is ladled on to sieves, from which it runs into cooling 
vats, and is filled while moderately hot into clean tubs. 

The liquor is clear and of a light-brown colour for the 
best XX quality, and darker for the common. The coolers 
or setting backs are of wood or zinc, and the liquid is not 
kept hot longer than it can be helped, as it is liable to turn 

Stephan remarks that ** in the preparation of size, steam 
is usually let direct on to the materials, so that considerable 
dilution takes place **. In his patent (English patent, 5,164, 

1898) the size is placed in a boiler with double walls surrounded 
by steam ** which does not come in contact with the size". 
The heating is done under pressure, and an arrangement 
is introduced for accurately measuring the size as it is 
passed into the troughs. A later patent (2,059, January, 

1899) gives as an improvement ** the employment of one 
or more suitable shaped discs . . . upon the shaft of the 
boiler ... for the purpose of separating the various stages 
in the boiling of the sizing material, and causing the latter 
to pass from one stage to the other gradually, according to 
its development '\ 

** Blue size," used by shoemakers as a first dressing for 
leather to fill up the pores and make an even foundation 
for blacking, was once an important article of the trade. 


and several firms made it a specialty. It is still made, 
but is not so much in favour as formerly. It consisted of 
a size, the colour not being of much importance, dyed 
heavily with logwood, with the addition of a small propor- 
tion of alum or ferrous sulphate, to produce a bluish black. 
It has been replaced by ** chemical dyes ". 

Mitscherlich patents a paper size made of horn cuttings 
dissolved in dilute alkali, with rosin added as long as it will 
dissolve. It is a transparent liquid which sets to a tough 
jelly when cold. 

'' Distemper " is whiting or sulphate of lime mixed with 
size and a pigment. The size is sometimes putrid, or be- 
comes so in a damp place. Various antiseptics, such as 
salicylic acid, have been mixed with it, but it is generally 
sufl&cient if the size is good, and the drying not delayed. 

" Concentrated size " is sold in packets for export. 

Special Prepared Glues. 

A large number of additions have been made to glue, 
many of them patented and highly recommended, but in 
most cases they have not answered the expectations of their 
inventors. It may be said generally that it is best, for 
nearly all purposes, to employ good glue without any ad- 
mixture. The uselessness of powders has been already 
alluded to. The effect of chemicals is almost invariably 
to diminish the adhesive power or the consistence. 

Mineral salts are liable to the following objections : — 
1. Those which are efflorescent, like most salts of soda, 
creep up to the surface, and occasion a " bloom " or ** frost " 
on the cakes or size, by forming a crust of small crystals. 
A similar fault attaches to sal ammoniac (ammonium 
chloride), which is frequently used to prevent frosting or 
cracking, after drjnng, in glues containing this substance : 
2 to 5 parts glycerine is proposed to be added to 100 parts 


boiling glue or size water, for every 15 parts of ** sal 
ammoniac or its substitute " (English patent, 14,936, 1895). 

2. Deliquescent salts, like chloride of calcium, tend to 
keep the glue damp and more susceptible to atmospheric 
conditions and to moulding. Chloride of calcium and zinc 
salts increase the adhesion to smooth non-porous surfaces, 
but at the same time diminish the drying power and tena- 
city. Homdam of Diisseldorfif (German patent, 22,269, 
1882) adds 8 to 10 per cent, of calcium or magnesium 
chloride, as an intended preservative, a purpose for which 
these salts do not act satisfactorily, and 30 per cent, to 
obtain a liquid preparation. 

Many of these substances, such as sugar, etc., should be 
looked upon as weighting materials or adulterations. 

Addition of Powders to Olue and Gelatine. 

This practice has' been described in chapter i., p. 17. 
Various objects have been assigned to it. In many 
varieties, some of them patented, white lead, oxide of zinc, 
and other powders are admixed, with the stated idea of 
forming a chemical compound with the glue, and thereby 
increasing its strength. That little or no combination does 
occur is proved by the fact that the insoluble powder can be 
extracted unaltered, with the exception of a small proportion 
which may have been dissolved by the acids or other sub- 
stances contained in the glue. Nelson's ** patent opaque 
gelatine " is whitened by the addition of a small quantity of 
finely divided carbonate of lime during the preparation, the 
effect being to give the jelly made from it the milky or 
opalescent appearance that is often preferred, besides, as in 
the Eussian glue, conveying a distinctive trade character to 
%e product. In the small quantity used, the addition is 

It may be laid down generally that the clearer a glue 


appears, and the more free from clouds, streaks or spots, the 
more care has been exercised in its preparation, and the 
better is likely to be its quality. 


The precipitate produced by tannin in a gelatine solution 
is sometimes supposed to be analogous to leather, but is 
quite different in properties. Hide after tanning is absolutely 
insoluble in water, but tanno-gelatine has a certain amount 
of solubility which depends on the kind of tannin, on the 
temperature, and on the other substances present. More- 
over, leather is hard and consistent, whereas tanno-gelatine, 
until it has thoroughly set, is soft and adhesive. This 
character of tanno-gelatine has actually been made avail- 
able in its use as a special cement, on account of its property 
of adhering firmly to leather. The Papier Zeitung (xviii., 
p. 2,618) gives the following formula for joints in leather 
driving belts : — 

" Good hide glue and American isinglass in equal parts, 
after soaking for ten hours in water, are heated with pure 
tannin till the product becomes sticky. The ends should be 
roughened and the cement applied hot.** 

With the object of securing adhesion to metallic surfaces 
many mixtures have been proposed. Some of them contain 
sulphur, such as " BuUer's Glue Compound,*' which is made 
from common glue, sulphur, and Paris white (English patent, 
17,833, 1890). 

Chrome Glue. 

When glue solution is treated in the dark with soluble 
chromates, such as bichromate of potash or ammonia, or 
with chromic salts like chrome alum, no change takes place ; 
but if exposed to sunlight, the glue is rendered insoluble in 
water. This property has been used in the ** carbon pro- 
cess" of photography. After exposure to an image the 


plate is soaked in water ; the parts unaffected by light are 
dissolved, while those that have been acted on are left in 
relief. The picture, after hardening with alum, can be 
copied by electrotyping, impressed on' soft metal (Wood- 
burytype), or even printed from direct with lithographic 
ink (Collotype). 

The reaction is also apphed to the production of water- 
proof glue and coatings for fabrics. 

An application of the compound of chromic acid with 
gelatine is suggested by Izard (Comptes Rendns, cxviii., p. 
1,314). With a view to prevent the tarnishing of silvered 
surfaces, and especially the mirrors of astronomical tele- 
scopes, he coats them with a very fine film of bichromated 
gelatine, which is said to be extremely adherent, transparent, 
and lasting, and to furnish complete protection even under 
the action of gases like sulphuretted hydrogen. The pro- 
cess was successfully used on telescope mirrors at the 
Toulouse Observatory, and did not injure the definition. 

Blasting "Gelatine". 

The so-called gelatine explosives do not contain gelatine 
at all, but are made of various mixtures, whose basis is 
some form of soluble nitrocellulose or collodion cotton, 
therefore lie outside the scope of these articles. The name 
is derived from their horny consistence. 

Flaked Gelatine. 

With the object of promoting the easy solution of gela- 
tine or glue, E. S. Spencer grinds it in a mill, sifts out the 
powder, and passes the coarser granules between smooth 
rollers, which may be gently warmed if necessary, till they 
are flattened out into thin flakes, which will then be readily 
soluble, and if well dried and carefully kept, are not aflFected 
by climate (Enghsh patent, 14,491, 1893). 


Size, glue, or gelatin (Cannon, Lincoln, English patent, 
10,596, 1891), are scented with various aromatic or anti- 
septic substances. 

Coloured Papers. 

A large quantity of the lighter glues were formerly 
employed in the manufacture of coloured paper y an industry 
that after 1870 received a great artificial impetus in Ger- 
many, with the usual result that the modern fast-working 
machines produced huge quantities, and caused a glut in the 
market, which led to the closing of about half the firms in 
this trade, with a corresponding cessation of the demand for 
the glue products. In 1896, according to HandeVs Musev/ni, 
there were in Germany fifty-five manufactories of coloured 
paper, many of which are in a precarious position, owing 
not only to fluctuations in prices, but to a general depression 
of the whole industry. 


Gelatine in aqueous solution is precipitated by formal- 
dehyd, H.COH, or CHgO, as a substance which on drying is 
a white powder, neutral, inodorous and insoluble in water 
and dilute chemical reagents ; it is used as a dressing in 
surgery. In conamerce, formaldehyde is met with in aqueous 
solution as " formalin,'* containing 40 per cent, of CHgO. 
If 1 cc. of formalin be added to 200 cc. of a 5 per cent, gela- 
tine solution, the latter is changed into a gelatinous mass, 
h (^ not melted by heat nor reduced by wa ter. If a smaller 
quantity of the formahn is added (1 in 1000) the jelly is 
said to be meltable, but with a higher tenacity ; when dried 
it becomes insoluble in warm water. Formalin in less pro- 
portion, though it does not prevent the dried gelatine from 
dissolving in warm water, is said to improve the **body" 
of the jelly and its keeping qualities, and also the tenacity 
of the glue. The following table is given of the properties 



of a 5 per cent gelatine solution containing various propor- 
tions of the reagent formalin. The third column indicates 
the bearing weight of the jelly, exclusive of the weight of 
the disc (140 grammes) used as a sinker. 

Percentage of 

formalin in 

the jelly. 

Proportion of 


tx> 100 parts 

dry gelatine. 

Bearing weight 

of the jelly m 


Behaviour ou 


after drying on 

a glass plate. 





























r Remains a jelly i 
\ on warming / 







































[?] 796 



The results show that up to 1 per cent, of formalin the 
solidity of the jelly increases ; above that it declines. Up to 
0'02 per cent. (1 in 5,000) it re-dissolves in water after drying. 
Even with this small proportion the firmness of the jelly 
has been sensibly increased (147 to 173). With a 10 per 
cent, gelatine solution it had increased from 394 to 587. 
The English patent, 4,696, 1894, claims the addition of for- 
mahn during the manufacture of size and glue in such pro- 
portion that the product shall liquefy in warm water. 

In examining commercial sheet gelatines for printers^ 
and photographic use, we have often found small quantities 
of formalin present. It seems to improve the quality, a 
very small percentage increasing the tenacity, flexibility and 
keeping qualities, while not affecting the transparency or 
rendering the material acid. When applied to articles that 


may be used as food, a strength of 1 in 50,000 in the final 
prepared product is not injurious to health, but should not 
be exceeded (Kideal & Foulerton, Public Health, May, 1899, 
p. 668). 

Zimmermann (patent 8,391, 1897) applies dilute formal- 
dehyde mixed with glycerine, vaseline, oil or yolks of eggs, 
with or without flour, to the surface of photographic films, 
*' which are thereby rendered more pliable and not so hard 
as when the formalin is applied by itself '\ 

It will be gathered that formaldehyde in more than traces 
renders gelatine insoluble ; the product, moreover, is almost 
unaffected by water, is more or less hard and elastic, and, 
owing to the antiseptic properties of the formaldehyde, is 
nearly imperishable. In English patent 18,237, 1895, it is 
used for rendering glue and gelatine insoluble. 

Preparation of Formo-Oelatin (Vloten, Ghem, Zeity xx., 407). 

Vloten prepares formo-gelatin in fine powder by precipi- 
tating a dilute warm solution of gelatin with formaldehyde 
slowly added with continual stirring. The mixture is then 
beaten with an egg whisk, and the precipitate of formo- 
gelatin is slowly produced as a scum similar to coagulated 
albumin; which can easily be removed from the liquid. The 
dried product is described as a ** smooth powder which may 
be introduced into wounds without producing irritation *'. 
It is called OlutoL 

Unaltered Oelatin in Formo-Oelatin (Eomyn, Pharm. 

Weekblady xxxiii. [Ij , 1896). 

The antiseptic value of a sample of formo-gelatin would 
be diminished by the presence of unaltered or free gelatin, 
and in this paper the following tests are proposed for its 
detection : — 

(1) A portion of the sample to be tested is heated vdth 


twenty times its weight of water for ten minutes at 100° C, 
and filtered while hot. If gelatin is present the filtrate will 
solidify when cooled. 

(2) 0*5 gramme of the sample is treated with 6 cc. of dilute 
caustic soda, ammoniacal silver nitrate is then added, and in 
the presence of much unaltered gelatin no darkening will 
take plswje for some time, but fairly pure formo-gelatin will 
blacken in a few minutes. 

With regard to the first test the author has noticed that 
pure " formo-gelatin is slowly decomposed by boiling water 
as the extract gives the biuret reaction *'. 

Artificial Silk from Gelatin (Knecht, Dyers' and 

Colours J., 14 [12] , 252). 

An interesting use for formo-gelatin is the production of 
Vanduara silk, which consists of extremely fine threads of 
gelatin treated with formaldehyde. A solution of the gelatin, 
containing any necessary colouring matter, at a constant 
temperature is drawn out into fine threads and, by means of 
an endless band, these threads slightly twisted are wound on 
to reels. The *' fibre '' is then subjected to the action of 
formaldehyde gas for some hours. Vanduara silk is claimed 
to be equal in appearance to the best net silk, but of course 
it is not so strong. The imitation silk is affected by moisture, 
becoming limp, but it regains its normal strength when dry. 
It is described as being insoluble in alcohol, ether, alkahes and 
acids, and bums like genuine silk. The cost of production is 
favourable, as the dyed article can be produced at less than 
3s. 6d. per lb. 

The Detection of Formaldehyde (Formalin) in Olue or 


The following process for detecting the presence of for- 
maldehyde in glue or gelatin has been found reliable in the 


laboratory : A quantity of the sample, about 10 grammes, and 
an equal weight of water is placed in a Wurtz distilling flask 
and distilled by heating to 110° C. in an oil bath and passing 
steam into the liquid. The first 20 to 30 cc. of the distillate 
is collected and portions tested for the presence of formal- 
dehyde by (1) Hehner's milk test, adding an equal volume 
of milk (free from formalin), and then concentrated sulphuric 
acid : a rose-purple colour at the junction will appear if urS^iro 
part of formaldehyde be present. (2) Carbolic acid test : One 
drop of dilute carbolic acid and then cone. HgSO^ : a pink 
colour indicates formaldehyde. This test is exceedingly 
delicate. (3) Schifif's reagent. A dilute solution of magenta 
bleached by sulphurous acid has its tint restored by weak 
formaldehyde. But the test should be freshly prepared, is 
not very deUcate or reliable, and is given by other aldehydes 
and by many substances. It is, however, useful as a positive 
and confirmatory test. 

Liquid Glues. 

A very large number of receipts, and even patents, are 
recorded for the production of a glue that shall remain per- 
manently liquid, and not require dissolving or heating for 
use. It has long been known that the hardening of glue, 
or its gelatinisation, can be prevented by the addition of 
certain salts, or by the action of acids like nitric or acetic. 
But it will be easily understood that an agent which retains 
the glue permanently in a liquid form in the cold will also 
hinder its setting when it is used in the ordinary way, and 
in addition will probably reduce the ultimate tenacity, un- 
less the agent used were a volatile one, like the spirituous 
solvents used for resinous cements. But glue and gelatine 
are not soluble to • any extent in alcohol or ether, not at all 
in benzol, petroleum, and other hydrocarbons, and only 
form an emulsion with oils. For this and other reasons 


the greater number of the recipes for the preparation of a 
glue that shall remain always liquid, after the manner of 
an ordinary gum bottle, are of no practical value. 

One or two that have stood the test of long use and 
experience may be quoted : — 

Acetic-gelatin. This is one of the most valuable of this 
class of cements, as it remains semi-liquid and is easily 
made quite fluid by warming; it is transparent and very 
tenacious, so that it is sold under many names, such as 
** giant," ** diamond," ** Hercules" cement, etc., for uniting 
glass and china, and for bilhard tips. It can instantly be 
detected by the strong smell of acetic acid. Gelatin in fine 
threads is gently warmed and stirred with glacial acetic 
acid till dissolved. 

" Diamond cement " is properly isinglass and gum 
ammoniacum dissolved in alcohol, and is practically water- 
proof. It is also used for glass and china. 

If glue be dissolved in water, and one-fifth its weight 
of ordinary nitric acid be then added in small portions at 
a time, the solution will not gelatinise, but it still re- 
tains its adhesive powers. A very good liquid glue may 
be prepared by dissolving in a water bath 100 parts of 
glue in 250 parts of vinegar; 250 parts alcohol and 10 
parts powdered alum are then added, and the whole heated 
for a quarter of an hour, when it is ready for use. 

Another good liquid glue is made by heating for several 
hours on a water bath 12 parts glue, 32 parts water, 2 
parts hydrochloric acid, and 3 parts zinc sulphate, the 
temperature not being allowed to rise above 180° F. This 
is called on the Continent ** Knaffl*s glue " : it will keep 
good for a long period, and is largely used for joining a 
great variety of materials, such as wood, horn and mother- 

About 1895 a solution of chloral hydrate figured in a 


large number of patents without much reason. It is ex- 
pensive, and its efl&ciency as a bactericide and preservative 
is disappointing. When present to the extent of about 
10 per cent., it causes the gelatine to remain permanently 
liquid, hence is an ingredient of many elaborate recipes for 
liquid glues. 

An example of the extraordinary complexity of some of 
these recipes is seen in the following (English patent, 2,079, 
January, 1894) : — 

*' In 100 parts of water dissolve, successively, 


" Chloral hydrate 2 parts, 

Aramoniura sulphocyanide ... . 5 „ 

Boric acid 3 „ 

Zinc chlorate 0'6 „ 

Zinc sulphate 1 part, 

Glue 450 parts, 

stir frequently while mixing to prevent setting. The chloral 
hydrate and sulphocyanide produce the necessary degree of 
fluidity, while the desired tenacity and elasticity are pro- 
duced by the boric acid and the zinc salts.** It may be 
observed that if, as is usually the case, traces of iron were 
present in the glue or in the other ingredients, the liquid 
glue would acquire a more or less deep red colour, owing 
to the action of the sulphocyanide. The boric acid and 
zinc salts would act as antiseptics, but, hke many other 
recipes of like nature, it is more than doubtful if the results 
would justify the trouble and expense. 

Mineral glue (G. W. Bremner, London, English patent, 
4,035, 1882). ** Syrup of biphosphate of alumina, crystalUne 
or mineral gum or syrup obtained from bauxite or any or all 
of them are heated to boiling with steam, and refined alumina 
added, as much as the stuff will absorb. Dissolved bone, 
ivory, hoi*n, hair or ligamentous tissue is then added, and 
the whole boiled for several hours. A hard material that 


will polish may be made by boiling this glue with plaster of 

Liquid glue (Martens, Germany, English patent, 17,370, 
1896). Glue made from leather parings is softened with 
water . . . 100 parts of glue to 150 of water ... 10 parts 
of salicylic acid are added, and the mixture digested till clear. 
Oil of cloves is then added to the proportion of 1 gramme to 
each kilogramme of glue. The product, which remains liquid, 
is suggested as a gum substitute. 

Elastic glue. Glue and glycerine in equal parts. Re- 
mains permanently elastic and soft. 

Some examples may be given of glues made from other 

Liquid casein glue (Bolder, Charlottenburg, English 
patent, 19,661, 1897). A mixture of casein, borax and water 
boiled for half an hour. It is a thick liquid of great ad- 
hesive power, and has long been used for certain purposes 
by joiners and bookbinders. 

Waterproof glue (Gardner, English patent, 21,774, 1896) 
is a mixture of casein, albumin, blood, or gluten with the 
hydrates, carbonates, silicates, borates, or phosphates of the 
fixed alkalies. 

Oluten glue is more of a paste than a glue. It is a very 
sour mixture of gluten and fermented flour, and has very 
little adhesive power (Wagner). 

** Albumen glue " is a misnomer, as it contains neither 
glue nor albumen. It is partly decayed gluten from flour, 
washed with water and then heated to 15° or 20° C, when it 
ferments and becomes partly fluid (by bacterial action). It 
is then dried at 25° to 30° C, and " keeps any length of time 
in a dry place. Dissolved in twice its weight of water it 
can be used as glue for all purposes." 

98 glue and glue testing. 

Puncture Fluids for Pneumatic Tyres. 

A large demand has arisen for this special use. The 
requirements are somewhat exacting. It is necessary that 
the cement, carried in a small bottle, should be easy of 
application on an emergency, and should not be liable to 
deteriorate or become mouldy. It should remain liquid in 
the bottle, or should have the capacity of easy liquefaction 
by gentle warming, with the power of forming an elastic and 
tenacious plug. Patents are numerous. One class are based 
on india-rubber, gutta-percha, or gum resins, with spirituous 
or hydrocarbon solvents. These are not affected by water, 
but are said to injure the material of the tyres and to pre- 
vent a permanent repair ; they are often also deficient in 
tenacity. Another section, though not quite waterproof, are 
excellent for temporary use ; they are founded on glue with 
glycerine as a softener ; for instance, glue, glycerine, water 
and chloroform (both the glycerine and the chloroform are 
antiseptics and render the preparation unalterable). Glue 
and glacial acetic acid is dangerous, although a strong 
cement, on account of the metal fittings. If too much glue is 
used in these recipes, the coat will be stiff and liable to crack. 

A coat of Brunswick black will waterproof the mend for 
a time and prevent cracking. It must of course be allowed 
a little time to harden. 

A *' Glue Composition for Tyres, etc." (patent 18,864, 
1896), called a ** gelatinous resiUent composition,'* is ** an. 
admixture of glue, sulphur, sulphate of barium, alum and 
collodion, with sulphuric, acetic, nitric and formic swjids. It 
can be softened with glycerine.*' 

Culinary Gelatine. 

The value of gelatine as a food has given rise to much 
controversy, even to the extent of denying that it has any 


value at all. On the other hand, popular opinion is apt to 
judge of the strength of a soup by its forming a stiff jelly 
when cold, and jellies, such as '* calves* feet," are said to be 
very nourishing for invalids.^ By physiological experiments 
it has been shown that ** although in diet it will replace a 
certain amount of the true proteids," — fibrin, albumin, and 
casein — *' acting as a * proteid-sparing * food, it cannot 
altogether take the place of proteids. Animals fed on 
gelatine instead of proteid waste rapidly" (Halliburton,. 
Essentials of Animal Physiology, 1899). 

Meat extracts, which are valued for their stimulant and 
not nutrient properties, are expected to contain only a small 
amount of gelatine. This is attained by digesting the com- 
minuted fibre in cold water, when the gelatigenous tissue will 
not dissolve. Allen (Commercial Organic Analysis, vol. iv., 
1898, p. 306) gives a table of analyses of conmiercial meat 
extracts, from which it appears that the gelatine varies from 
0*25 to 5*50 per cent. : many of these have obviously been 
prepared by a hot process. The objection to gelatine in these 
preparations is that it gives a spurious solidity and conceals 
an extra amount of water. 

For culinary purposes the form in thin sheets is commonly 
placed under the crust of raised pies, etc., to melt down 
gradually. For soups a number of varieties are made — thread, 
cut, and fancy gelatines ; some are nearly white, and bear a 
close resemblance to isinglass ; in fact they are sometimes 
sold as ** patent Isinglass ". 

^I have found in three "Invalid Jellies" in the market the following 
amounts of gelatin : Brand's, 5*15 ; Mason's, 5*46 ; Maynard's, 4*11 per 



An examination in our laboratory of a number of com 
mercial kinds gave results as follows : — 

Characters of the 10 per cent. Jellies. 





No. 1.1 

No. 2. 

No. 3. 

No. 4. 


Colour . 














Appearance . 








Insoluble matter . 

trace of 

trace of 

trace of 

trace of 


white — 

white — 





trace of 








Odour (hot) . 








„ (cold) 








Liquefying pt. C°. 
















Units of water 

absorbed — 1 hr. . 








,, „ 48 hrs. 








Consistence cold . 
























Composition of the Original Gelatins per cent. 

Water . 


Organic Matter . 





















100 00 







Nelson's Patent Gelatine is rendered opaque by a small 
quantity of chalk. It is stated to be prepared by washing 
skin parings, etc., and digesting them in dilute sodium 
hydroxide solution for ten days. They are kept at a 
temperature of 70° F. in an air-tight vessel, and afterwards 

' No. 1 was found to be *' free from ohondrin ; suitable for culinary, but 
rather soft for photographic use, unless mixed with a ' hard ' gelatin ". 

For ordinary photographic emulsions Abney recommends a mixture of 
*'hard" and *'soft" gelatins in proportions dependent on the weather, *'& 
good mixture being 1 part hard to 8 soft ". 


thoroughly washed in cold water. They are then bleached 
by sulphurous acid, as described in Chapter II., again 
washed and converted into gelatine by heating with water. 

Swinboume*8 Gelatine (English patent, 9,033 and 11,975, 

Hides, skins, or glue-pieces free from hair, reduced to 
shavings or slices, soaked five or six hours in cold water, 
then the water changed, and so on for some days until no 
smell or taste. If intended for soup, the shavings are 
merely dried, and are then ready. If for gelatin, they are 
heated with water, not above boiling, strained through the 
filter cloths, and run on to slate or other material to dry. 
They cut up similarly cod-sounds or other fishy matters. 

Bones yield excellent culinary gelatine, clear, transparent, 
inodorous, and nearly tasteless. 


is not a gelatin, but a collagen which is very easily con- 
verted into gelatine by heating with water. It is properly 
prepared by slicing and drying carefully the swimming 
bladder or ** sound*' of the sturgeon {Acipenser vulgaris), 
but many other fishes will yield an articlejittle inferior. The 
commercial article occurs in fine yellowish-white threads, 
translucent and tenacious, free from odour, and having a 
very faint-- fishy taste. 

It is distinguished from gelatine imitations by soaking in 
warm water and examining under the microscope, when true 
isinglass will show a network of long curling fibres, while 
gelatine will be simply hyaline. In hot water isinglass dis- 
solves almost entirely, and gives a very firm jelly on cooling, 
strongest in the ** Eussian '' variety, from the sturgeon. 

** Brazilian" or ** Cayenne" isinglass, from the fish 
Silnrua Parkeri, is met with in sheets an inch or more 
in thickness. 



*' Eat 's-tail " isinglass, from the sound of the cod, hake 
and other fishes, is opaque and incompletely soluble. 

For soups, jellies, etc., a good quality gelatine is prac- 
tically equal to the more expensive isinglass, and is 
frequently sold as such : it takes rather longer to prepare. 

F. ProUius (Dingler's Polyt, J., ccxlix., 425) gives the 
following determinations in various foreign samples. For the 
viscosity, 1 part of the specimen was dissolved in 90 parts 
water : — 

Time re- 




quired for 

solution to 

run out. 

Astrachan, from Schmidt & 

Per cent. 

Per cent. 

Per cent. 


Dihlmami, Stuttgart 





Astrachan, from a collection 





Astrachan, fine iridescent 

Russian quality, Tubingen 

collection .... 





Astrachan, Bussian, from 

Gehe of Dresden 





Astrachan, in laminae from 






Astrachan, in threads, known 

as Hamburg threads . 





Hambuig isinglass 





Another quality . 





Boiled northern, fish bladder 





Icelandish bladder 





Indian isinglass . 





Yellow, quality unknown 






The use of isinglass, especially the commoner kinds, for 
clarification, is due almost entirely to the fine fibres already 
mentioned, which entangle and carry down suspended 
matter, therefore in this case gelatine will not act as a 
substitute. For beer the finings are supplied ready-made, 
and consist of Brazilian isinglass dissolved in sour beer with 
a variable addition of sulphurous acid or calcium bisulphite. 
They are often very foul in character. Inasmuch as the 
addition of *' finings '* to liquors has often been made a 


cloak for adulteration, the maximum amount that may be 
added, and its nature, is in most countries prescribed by the 
excise. On a large scale the isinglass after soaking is reduced 
to a fine state of suspension in the water or other liquid by 
pressing with revolving brushes through sieves. 

Isinglass Substitute (**Isinglassine"). 

The gelatinous material from calves' feet and other 
sources is reduced by machinery to a pliable homogeneous 
mass, rolled out into sheets, dried, pressed and shredded ; 
each round is cut into about 125,000 shreds or staples. 

Many lichens and seaweeds give a jelly on boiling which 
is used in food and sometimes in medicine. The substances 
causing the gelatinising property are mostly non-nitrogenous 
and are related to cellulose. 

Irish Moss {Chondrus crispus) contains ** Carrageenin," 
allied to pectin. It has been employed as a substitute for 
isinglass, as a size, for thickening colours in calico printing 
and in stiffening silk. 

Gelose, from Oelidiwm corneum, or Chinese moss, is met 
with in commerce in long thin threads called '* Japanese 
isinglass," but although it is said to have ten times the gela- 
tinising power of the real article, it is not a suitable substi- 
tute since the melting point of the jelly is above the 
temperature of the mouth. (Allen.) Agar-agar is a similar 
substance sold in long hollow prisms, and much used for high 
temperature cultures in bacteriology instead of gelatine plates, 
which would become liquid above 26° C. 

Algin or alginic acid is a gelatinous body first isolated by 
Stanford from Laminaria and other seaweeds. Although 
nitrogenous, it is not a proteid, as will be seen by comparing 
the formulae. 

Gelatin (Schutzenberger) C7gHi24N24029. 
Algin, C76H80N2O22. 

104 glue and glue testing. 

Medicinal and Other Uses. 

Gelatine is thus mentioned in the British Pharmacoposia 
of 1898 :— 

** Oelatinurriy Gelatin. — The air-dried product of the action 
of boihng water on such animal tissues as skin, tendons, 
Hgaments and.bones. 

" Characters and Tests. — In translucent and almost colour- 
less sheets or shreds, a solution in 50 parts of hot water is 
inodorous, and solidifies to a jelly on cooling. Gelatin is 
insoluble in alcohol (90 per cent.) and ether. It dissolves in 
acetic acid. Its aqueous solution yields a precipitate with 
solution of tannic acid, but not with solutions of other 
acids, nor with solutions of alum, lead acetate, or test-solu- 
tion of ferric chloride." 

The definition would seem to exclude fish gelatine. The 
last paragraph demands the absence of chondrin. Gelatin is 
made official for the preparation of glycerine suppositories. 

The appendix prescribes : — 

** Solution of Isinglass. 

" Isinglass, in shreds, 4 grammes. Distilled water, warm, 
sufficient to produce 200 cubic centimetres. Mix, and digest 
for half an hour on a water bath with repeated shaking, and 
filter through clean moistened tow. Solution of isinglass 
must be recently prepared." 

It is used for testing the freedom of gallic acid from 
tannic. Ordinary gelatine answers equally well. 

Two unofficial preparations are : — 

Vinum Xericum detannatum, and Vinum Aurantii 
ditto : detannated sherry and orange wines. Half an ounce of 
gelatine, cut small, is macerated with on« gallon of the wines 
for fourteen days, and decanted clear from the insoluble 
tanno-gelatine. The object is for preparing various medi- 
cated wines with drugs that would be precipated by tannin. 


** Lamellae *' are discs of gelatin with some glycerin, 
weighing ^ to ^V grain, and containing a minute dose of a 
powerful alkaloid, to be dissolved in water for ophthalmic 
and hypodermic use. Four kinds are specified in the British 

" Glycerin jelly " is a solution of gelatin in glycerin used 
for mounting microscopic sections. A semi-solid mixture of 
the same is official as Olycerin suppositories ; also in the 
Throat Hospital pharmacopoeia in two different consistencies 
as Oelato-glycerin and Glyco-gelatin, the former used for 
nasal bougies, the latter for medicated lozenges. Glycerin 
jelly is also used for the hands. 

Gelatin capsules, for nauseous drugs, and gelatin-coated 
pills are well known. 

Minor uses of gelatin are in confectionery, and in coloured 
sheets for toys. Deeply coloured sheets of this kind were 
successfully introduced some years ago in London for colour- 
ing theatrical limelights, and were found when mounted in 
frames to be much lighter, less expensive and fragile, and 
more easily renewed than the glass plates previously used : 
they are not much aflfected by the heat, especially if a cell of 
alum solution is interposed, as is done in the cinematograph. 

Use of Oelatin in Bacteriology. 

Of late years a considerable demand has arisen for the 
finest qualities of gelatin for purposes of bacteriological 
research. The gelatin employed in this work must be 
quite clear and bright, any opalescence such as is present 
in Nelson's being very objectionable, it must also be fairly 
neutral, and especially of high gelatinising power. Hitherto 
this demand has been supplied almost entirely from the 
Continent, but latterly English firms have turned out 
gelatin quite equal if not superior to foreign makers, for 
this purpose. 


As already pointed out in describing its manufacture, 
most bacteria, under certain conditions, find gelatin a 
medium for rapid and luxuriant growth. For bacterio- 
logical work the gelatin obtained in the form of sheets 
or thin strips is made up to a 10 to 20 per cent, solution 
with meat broth, and this nutrient jelly, which is obtained 
by clarification with albumen perfectly clear and bright, 
forms a most useful medium for the cultivation of micro- 
organisms; it allows any growths to be easily examined 
under the microscope, and the jelly can be remelted, and 
moulded to any required shape, without the danger of 
killing the bacteria by overheating. Moreover, it affords 
an important distinction between various species as to 
whether they are capable of permanently liquefying the 
gelatin or not. 



Much controversy, especially in Germany, has centred on 
the large number of methods that have been proposed for 
glue testing, and there is no doubt that difficulties arise in 
the interpretation of results, and in obtaining absolute 
measures, or figures that shall be comparable between 
different observers. Therefore the necessity for agreeing 
upon standard methods of working, or at least of stating 
in each case the exact details of procedure. Systematic tests, 
however, are of use to manufacturers in controlling their 
processes, and to users in avoiding loss and mistakes in 
purchasing, owing to the fancy prices and misleading names' 
and description^requently put forward. 

Julius Fels (Chem. Zeitung, xxi., Nos. 8 and 9, 1897) 
gives the following list of methods, to which we annex his. 
criticisms : — 

A. Chemical Methods, 

1. Grager. Dingler's Polyt J., 1852, 126, 124. 
Tannin Titration. " The tanno-gelatine is of uncertain 

2. Noffat. Wagner's Jahresb,, 1867, 672. Nitrogen by 
soda lime. ** Other nitrogenous compounds are present." 

3. C. Stelling. Chem, Zeit., 1896, xxV, 461. Estima- 
tion of matters not glue (after pptn. by tannin or by 
alcohol). ** Inaccurate." (See F. Gautier, Zeit. f. AnaL 

Chem,,, xxxiii., 4.) 



B. Physical Methods. 

4. Schattenmann. Dingler's Polyt. J., 1845, xcvi., 115. 
Absorption of cold water on soaking. ** Uncertain." 

5. Cadet. Absorption in damp air. " Unreliable." 

6. Lipowitz. Neue Ghera.'techn. Unters., Berlin, 1861, 
37-42. Consistency and bearing weight of jelly. " Correct 
as a comparative method." 

7. Weidenbusch. Dingier 's Polp. J„ 1859, 152, 204. 
Breaking strain of a rod made of glue and plaster of 
Paris. ** Uncertain." 

8. Artillerie Werkstatte Spandau. Weight required to 
separate two pieces of wood glued together: '* Depends on 

9. Fels himself recommends determining the viscosity 
of a 15 per cent, jelly by Engler s viscosimeter {Jourii, Soc. 
Chem. Ind., 1890, p. 654). 

Chemical Examination. 

The determination of water and ash is necessary in order 
to render the other results comparable, and to assist in 
detecting adulteration. Ordinary glue of good quality con- 
tains from 12 to 18 per cent, of water ; a low result generally 
is a bad sign, as overdrying is known to injure the tenacity, 
while a high percentage diminishes the keeping qualities, 
besides adding spuriously to the weight. The ash is usually 
from 1*5 to 3 per cent., and its amount does not discriminate 
between hide and bone glues. It will obviously be much 
higher if mineral fillings have been added or the source is 
impure. The qualitative examination of the ash on the 
other hand gives a clue to the origin, as the ash of bone-glue 
fuses, its solution is neutral, and it contains both phosphoric 
acid and chlorine, whereas ash of hide-glue does not fuse, 
because it contains lime ; it is alkaline, and nearly or quite 
free from chloride or phosphate. 


After breaking in small pieces, a sufficient quantity of 
the glue is either reduced to shavings in a machine, or is 
pounded in a mortar, the pestle passing through a perforated 
cardboard guard to prevent scattering. One or two grammes 
of the coarse powder weighed in a platinum dish are dried 
first in the water oven, then at 110° to 115° C. till the weight 
is constant. The loss is water plus a small quantity of other 
volatile matter. Cautious incineration at a low temperature 
gives the ash, which should dissolve almost completely on 
warming with dilute nitric acid, the solution being tested for 
chlorides and phosphates, and also for other substances if 
necessary. The presence of more than a small amount of 
salt deteriorates the quality and appearance of glue. An 
excess of chlorine may also indicate that chloride of lime 
has been used as a bleacher in the manufacture. Various 
earthy powders added to glue during manufacture will be 
found in the ash. White lead, lead sulphate, zinc oxide, 
barium sulphate, and even lead chromate have been used. 

Reaction of the Glue. 

If a glue is acid, about 10 grammes of the powder are 
dissolved in warm water, and divided, into two portions ; in 
one the total acidity is estimated by standard alkali, the other 
is evaporated on the water bath, redissolved, and the fixed 
acidity determined. The diflFerence gives the volatile acids. 
As the nature and amount of the acids present in glue is 
often of importance, a closer examination may be made by 
distilling 30 grammes in 100 cubic centimetres of water by 
a current of steam, when the volatile acids will pass over. 
The distillate should be examined for free sulphurous acid, 
as it is sometimes present, having been used for preserving 
or bleaching, and its presence is generally objectionable. In 
leather-glues the volatile acids are about 0*02 per cent., in 
bone-glues from to 0*5 ; the higher amounts, with a smell 


of acetic or butyric acid, indicate that the jelly has been 
soured during the manufacture, and that the glue is unsuit- 
able for many purposes. Eisshng states that glue solutions 
which do not alter litmus may yet react with phenolphthalein, 
*' probably owing to an organic acid ". He estimated this 
with soda, but did not ascertain its nature. 

Any undue quantity of acid can be roughly detected by 
the taste. Glues may be alkaline from the addition of excess 
of lime in the manufacture to correct the sourness of the jelly. 
A large manufacturer informs us that ** a little acid or a little 
lime does not hurt, though it is much better neutral. We 
find that our customers prefer a glue that shows a little lime, 
rather than any acid, although an acid-made glue is not 
nearly so apt to decompose as a lime-made glue." Most of 
the bacteria which cause changes in glue thrive better in 
alkaline solutions, and are actually antagonised by acids. 
For uses, where colours are concerned, the glue must be 
neutral to litmus; for adhesive uses this does not matter, 
unless the alkalinity or acidity is due to defective prepara- 
tion. When the solution of glue in hot water rises higher than 
100° C, it begins to sour, and loses to some extent its property 
of forming a jelly. Sulphurous acid and sulphites are said to 
cause light patches on dyed wool. Kalmann states that the 
amount of sulphite can be determined, after titration of the 
acidity with soda and phenolphthalein, by means of iodine 
solution and starch, the organic matter not interfering. He 
mentions that for woollen manufacturers the glue must not 
exceed — free acid equal to 1 per cent. NagO, SOg 0*5, ash 3, 
water 15 per cent. 

Insoluble Matters, 

As extraction with water, filtration and weighing are 
somewhat tedious, a comparative result may be obtained 
by dissolving 30 grammes, making up to 1 litre in a 


graduated cylinder with hot water, allowing to stand in a 
warm place for twelve hours, then noting the volume of the 
deposit as " foreign matter," stating the quantities taken. The 
solution may be used for other tests. 

Determination of Gelatine. 

The organic matters of glue are very complex. Among 
the nitrogenous matters to be found in it there are probably 
several varieties of gelatine aud chondrine, as analyses of 
these substances, presumably pure, show differences in ele- 
mentary composition, in addition to the variations in physical 
properties. Products of change, such as albumoses and 
peptones are usually present, with small quantities of bases, 
sugars, and ill-defined bodies. For this reason the content 
of nitrogen and the various processes of precipitation by 
tannin, etc., yield results which have little relation to the 
commercial value. Gelatine has a greater power of adhesion 
and gelatinisation than chondrine ; the latter, however, is 
largely employed for size. 

The Bisler-Beumat method of tannin titration is briefly as 
follows : Two solutions are prepared : (a) 10 grammes of 
pure tannin to the litre ; (6) 10 grammes of pure isinglass 
and 20 grammes of alum to the litre. The latter is 
added to a measured quantity of the former till no further 
precipitate is produced ; the volume used is read off on the 
burette. The same volume of tannin is then precipitated by 
a 1 per cent, solution of the glue ; the relation between the 
volumes of glue solution and of isinglass gives the ratio of 
the glue to the sample of isinglass taken as a standard. 
Different specimens of isinglass and of tannin themselves 
show different ratios of precipitation ; 42*7 parts of gelatine 
are said to be equivalent to 57*3 of tannin, but the precipitate 
is somewhat variable in solubility and composition. Another 
process is to add tannin in excess, and estimate the amount 


of tannin left in solution by running in standard permanganate 
as long as it is decolorised. The proteids can be more exactly 
precipitated by chlorine (Eideal& Stewart, -4 Tiaii/s^, September, 
1897), or by bromine (Allen & Searle, ibid,, October, 1897). 

The presence of chondrine reduces the value of gelatine, 
especially for photographic emulsions. It may be detected 
by adding a concentrated solution of chrome alum to the 
glue or gelatine, dissolved in 10 parts of boiling water; if 
any injurious quantity of chondrine be present, the liquid will 
set while hot. 

Fat is very objectionable in glue, and for many purposes 
must be entirely absent. It renders the hot solution turbid, 
and on standing collects at the top as a greasy film, which 
may be removed by ether when the jelly cools. A manufac- 
turer's " grease test " is described as follows : " To a solution 
of glue add a little lampblack or Turkish red ; thoroughly 
mix with a brush, and paint it on some pieces of paper ; if 
there is an abundance of grease, it will show by little round, 
white, smooth surfaces on the red or black paint. Specially- 
required for the paper trade and some others." 

To determine the fat exactly, 20 grammes of the broken 
glue are treated with 50 cc. of water and 5 cc. of hydro- 
chloric acid (to decompose any soaps). When the glue has 
dissolved, sufficient calcium sulphate or clean sand is added 
to take up the water, and the mass is dried for some hours at 
105° C, and carefully granulated. It is then extracted with 
ether in a Soxhlet apparatus for five to six hours, and the 
extract dried and weighed. Kissling uses petroleum ether 
for extracting the fat. 

Stelling's process for determining ** matters not glue" 
consists in adding to a solution of 1 part glue to 4 parts of 
water alcohol of 96 per cent, in small quantities at a time, 
constantly stirring. Kissling says of this method : " We 
take 15 parts of glue and 60 parts of water, and make up 


to 250 cc. with alcohol, then determine the proportion of 
non-gluey matter present by evaporating down an aliquot 
part of the alcohol solution, previously filtered, and weighing 
the residue. The process is thus based on the supposition 
that alcohol of about 72 per cent, in volume leaves the 
gelatine intact, while it dissolves the products of decom- 
position, considered as non-gluey material.'* 

But gelatine is not entirely insoluble in alcohol of 72 per 
cent, inasmuch as, on evaporating down the filtered alcoholic 
solution, we obtain a residue consisting principally of gelatine 
and possessing a strong adhesive power, almost equal to that 
of the original glue employed. 

Nevertheless it cannot be denied that Stelhng's process 
has a certain value — very small, it is true — for the valuation 
of glues ; since, when we have to deal with products of a 
superior quality, the substances soluble in alcohol are in 
quantities much smaller than are present in ordinary glues. 

Saponification Process. 

W. Fahrion {Zeit Angeh. Ghem., 1898, p. 529) evaporates 
3 to 5 grammes with 15 to 25 cc. of an 8 per cent, solution 
of alcoholic soda to dryness, washes with hot water into a 
separating funnel, acidifies with HCl, cools and extracts 
with ether. The aqueous layer retains ** solid oxyacids,'* 
which are afterwards extracted by warm alcohol. The 
ether dissolves unsaponifiable matter, fatty acids, and 
'' fluid oxy acids '' ; the latter are insoluble in petroleum 
ether, the two former are separated by aqueous soda. 

He gives the following analyses of glue and glue-yielding 
substances : — 

Very pure white glue 






Solid. Liquid. 
0-04 0-27 




Animal hide powder 








Hide dressings . 








Horn of sheep . 








Bony part of sheep . 










Kissling considers that the above process '* may possibly 
throw light on the constitution of gelatine, but that it is 
not suitable for commercial work". 

Determination of Nitrogen Content. 

The determination of the amount of nitrogen in raw 
materials after washing, and in gelatines, is of great value. 
It is accomplished most easily by the Ejeldahl process. 

From i to ^ gramme of the material in small pieces is 
accurately weighed and is placed in a ''Jena glass" flask 
with long neck, supported on a concave piece of wire gauze 
over a Bunsen burner. A sand bath is not advisable, on 
account of the slowness, and danger of bumping. The 
flasks are preferably arranged in series in a slanting posi- 
tion on rails of iron rod in a good stink-cupboard : 20 cc. 
of sulphuric acid free from nitrogen, about 1 gramme of 
potassium sulphate (to raise the boiling-point), and a small 
globule of mercury are added. On applying heat at first 
there is danger of frothing or spurting, therefore the opera- 
tion has to be carefully watched; after a time it proceeds 
without attention if the heat is regulated. As soon as the 
liquid is only faintly coloured, it is allowed to become cold, 
then water is added cautiously, and it is washed into a 
Jena flask to a dilution of 600 cc, cooled, a piece of clean 
litmus paper added, then 50 per cent, soda solution, free from 
ammonia, till the litmus is blued, finally 1 or 2 cc. of pure 
strong sodium sulphide solution to precipitate the mercury 
and prevent its retaining ammonia, and three pieces of 
ignited pumice. Without delay the flask is connected with 
a condenser, and the contents distilled into a U-tube, con- 
taining 50 cc. of decinormal sulphuric acid, accurately 
measured, until 250 cc. has come over. This is then 
titrated with standard soda, using methyl-orange as in- 
dicator. The loss of acidity, due to neutralisation by 


ammonia, is thus obtained. Each cc. = 0014 gramme 
nitrogen. Multiplying by 5*56 -^ gives very closely the 
amount of gelatine, if other nitrogenous substances are 
not present. 

V. Schroeder and Paessler (Dingler's Polyt, J,, 287, pp. 
258, 283, 300) give as the^ content of nitrogen in various 
purified skins dried at 100"* C. 

Ox, calf, horse, pig, camel 

Goat and deer 

Sheep and dog 

Cat ... . 


17*8 per cent. 
17-4 „ 
17-0 „ 
17-1 ., 
18-1 „ 

It is very difficult to obtain gelatin pure and dry : Allen 
gives 17*9 as the percentage of nitrogen, I have found up 
to 18*2. But these differences are small when compared 
with the quantity found in other proteids, which is much 
lower, for example : — 

Albumen 15*7 per cent. 

Elastin 16*7 „ 

Fibrin 16-9 

Paraglobnlin (from blood) . . 15*85 ,, 

Keratin 16*8 „ 

On the other hand, many decomposition products con- 
tain a much higher percentage of nitrogen than gelatin. 

For commercial purposes the total nitrogen multiplied 
by 5 '56 is put down as gelatin or "real glue". Adding 
the ash and moisture and subtracting from 100 gives a 
remainder of ** other organic matter,'* which does not add 
to the strength. As examples of analyses on these lines 

^ This factor for gelatine is derived from the following figures obtained by 

different observers : — 

Per cent. N. 


Allen . . . . 






Schroeder & Paessler 



Bideal . . . . 




the results obtained in our laboratory with four samples of 
high-grade gelatins are appended, and it will be noticed 
that the figures for " other organic matter " are very small, 
amounting in only one instance to over 1 per cent. : — 

Nelson's. Coignet's " Gold 

A. B. "Extra". Label". 

Water 18-96 17-10 19*41 17-84 

Ash 4-19 3-80 1-50 1*47 

Gelatin (total N) (x 6*56) . . 76-73 78-26 78-34 79-67 

Non-nitrogenous organic matter 0-12 0*84 0*75 1*02 

100-00 100*00 100*00 100-00 

Adulterations and Admixtures. 

It has been already mentioned that mineral impurities 
will be found in the ash. 

If the figure obtained for " other matters," as described 
in the last section, be unduly high, gum, dextrin, sugar, or 
glycerin may be suspected. When a 25 per cent, solution 
of the material is precipitated by absolute alcohol, the sugar 
and glycerin remain dissolved, and are left on evaporation 
as a sweet residue, in which the sugar may be found and 
determined by Fehling's test, the difference being approxi- 
mately the glycerin. The matter insoluble in alcohol is 
boiled with dilute (1 per cent.) HCl, whereby gum and dextrine 
are inverted, and then, after alkalising, will reduce Fehling*s 
test on heating. 

Evers points out (Chem, Zeit, 1899, 23 [31], 333) that 
wheat gluten closely resembles animal glue in its behaviour 
to reagents, but yields with a small quantity of sodium 
chloride or magnesium sulphate a voluminous precipitate, 
whereas pure glue remains clear. 

Gelatine is sometimes itself used to adulterate other sub- 
stances. A small quantity added to chocolate has been found 
to raise considerably the percentage of water that can be 
introduced. [Onfroy describes a process for its detection : 


J. Soc. Chem. Ind,, 1898, 802.] For the same reason it is 
often added in undue quantity to meat extracts. When used 
for weighing or stiffening fabrics, its amount may be ascer- 
tained by extracting with hot water and determining the 
nitrogen, confirming also by other tests. 

Physical Tests. 

Glue should not become damp in ordinary air, or it is liable 
to turn mouldy. This fault would indicMe impurity or 
adulteration. The author has met with a few instances 
of weighting with coarse sugar, molasses or dextrine, in 
which this peculiarity was conspicuous. 

1. The appearance, hardness, and nianner of breaking 
furnish an idea of value which can be learnt by practice. A 
spUntery fracture shows that the glue is not well boiled. 
Glue from bones presents a milky appearance, due to a little 
calcium phosphate. Glue without gloss, very much warped, 
and of a very dark colour, may be excellent in adhesiveness 
and tenacity. 

2. Drying and Keeping Qualities, — About 10 cubic 
centimetres of a 20 per cent, solution are placed on a watch 
glass in a cool dry place, protected from dust, and observed 
for several days in comparison with a standard sample. 

3. Smell. — ^Varies very much. Leather glue smells least. 
Some do not smell when cold, but the hot solution is un- 
pleasant. This property is of importance for some uses. 
Standard glues may be kept in powder for comparison ; they 
retain their g«nell for years in closed bottles. The smell very 
often indicates the source : hide glue, pig's-foot, bone and fish 
glue all have a characteristic odour. 

4 Behaviour with Gold Water. — Schattenmann in 1845 
proposed the • absorption of cold water on soaking. Ten 
grammes of the sample in small pieces are placed in a 
weighed beaker, aud covered with 200 cc. of water, and 


kept covered at 60° F. for forty-eight hours. The water is 
then poured off and the remainder weighed. According to 
my experience, good glues will absorb about five and a half 
to six times their weight of cold water, and still exhibit a 
firm jelly. Some will even absorb eight to ten times their 
weight and still remain fairly firm. The more consistent 
and elastic in this state the greater the adhesive power, and 
the more water it absorbs the more economical in use. Bad 
glues will often become slimy and fluid in less than forty- 
eight hours. The water poured off should be free from 
putrid odour, and contain little in solution. The test is 
specially apphcable to bone glue ; that from animal offal does 
not show the same behaviour. 

KissUng soaks 100 grammes of the glue (apparently 
unbroken), supported on fine wire gauze, in cold water, and 
weighs at intervals of twenty-four hours, draining fifteen 
minutes before each weighing. In forty-eight hours he 
obtains numbers from 221 grammes to 332 granmies of 
water absorbed, his highest figure being 723 grammes in 
216 hours. This method seems inferior to the above. Still 
less can Cadet's method, absorption in damp air, be recom- 

Schlossmann {Papier Zeitungy xvii., 2484) takes strips 
of equal size (50 x 150 mm.), including a standard glue 
which is found to work well for the special purpose, makes 
two perforations with a hot rod, and suspends them by the 
upper perforation in graduated cylinders holding a litre of 
cold water, attaching a weight to the lower hole. At the end 
of every twelve hours, as long as the strips will hold together, 
they are carefully lifted and drained, and the volume of 
remaining liquid and its character observed. ** Good parch- 
ment and leather glues should be firm and sweet after two 
days in summer and four in winter.*' 

In America the absorption is usually stated in ounces per 


pound of dry glue. Thus ** a water test of 96 " means that 
1 lb. of glue has absorbed in twelve hours 96 ozs. of ice-cold 
water, equal to six times its weight. It is said, on the other 
hand, that with heavily evaporated glues, or those cut in 
very thick sheets, the water-test results are unreliable. 

5. Lipowitz' Method, 1861 : Consistency and Bearing 
Weight of the Jelly. — A 10 per cent, hot solution (the jelly 
from the 10 grammes in the preceding process, No. 4, could 
be made up to 100 grammes with hot water) is allowed to 
stand twelve hours at 18° C. in a cylinder of uniform width 
to gelatinise. A metal rod is soldered to the centre of the 
interior of a small tin cup, and is supported vertically by 
passing loosely through holes in two horizontal plates, one 
of which forms the cover of the cylinder, the other being 
fixed a httle higher. The upper end of the rod carries . a 
funnel, which can be loaded with fine shot. The cup is 
allowed to rest on the jelly, and the funnel is loaded until it 
sinks a certain distance into the jelly ; the weight of shot 
then gives the Lipowitz number. The cylinder should be 
surrounded by a water jacket to maintain a definite tem- 
perature. The result is also affected by the diameter of the 
cylinder and of the cup. Lipowitz gives the following 
figures : — 

Weight required to force 
Variety of Glue. the saucer down. 


Breslau 8-74 

Russian 8*18 

Cologne 2'67 

Muhlhausen 1 1*60 

Nordlingen 1-59 

Muhlhausen II 0*86 

The table from which the above figures are taken also 
contains the following : 1. Water 9 to 21. 2. The percentage 
of gelatine by tannin is in the same proportion as the water 
absorbed. 3. The weight supported by the jelly corresponds 


neither with the absorption nor content of gelatine, nor with' 
the price. (A reason for this has been given in the first 

A commercial form of the above, called the " shot-test,** 
is made by cooling the 10 per cent, solution in an ice-box for 
three or four hours till firmly set. A tube or small beaker is 
placed on the jelly and loaded with shot till it sinks. By this 
method I have found that a 10 per cent, jelly will bear a 
weight, ranging in different glues from 64 to 12 grammes, 
in bad cases falling to 0, the glue being hquid. Kissling 
ascertains how many seconds are taken by a glass, a zinc, 


Fig. 12. Kissling's Consistency Apparatus. 

and a brass rod, weighing respectively 50, 100, and 150 
grammes, and 8 to 10 mm. in diameter, to sink to a zero 
mark into a jelly. 

The apparatus he uses was originally devised for testing 
lubricating greases. 

The height of the support is so adjusted that the distance 
of the bottom of the pin B from the guide C is 100 milli- 


He soaks 100 grammes of the sample in 300 cc. of water 
overnight, warms for a short time next day till fluid, then 
maintains at 20° C. for four hours to set. " When the brass 



Leather glue 


Price in 

marks per 

100 kilos. 


»» »» 



»» »» 



»» f» 


» 4 


Mixed glue, 



)t ti 

6 . 



Bone glue, 




»i »» 



10 . 




rod takes 850 seconds, or longer, to sink, the consistency is 
100 ; while, if the glass rod sinks in less than one second, the 
consistency is 0.'* He gives the following examples : — 











He finds that skin-glues have a higher consistency of 
jelly, less odour, impurity, and volatile acid than bone glue, 
the water, ash, breaking strain, and absorption of water being 
about the same in both classes. But, like other observers, he 
can find no definite or constant relation between these factors 
and the price. 

I may here give a few notes from my own determinations 
of the consistency of jellies. As determined by the " shot 
test," the bearing weight is not the same as Kigsling s " con- 
sistency " or *' gelatinising power". The former is affected 
considerably by the skin which always forms to, a greater or 
less extent on the surface of the jelly. This can be avoided 
by slicing off, or by casting in a cylinder with false bottom 
and inverting, but it will be found that the jelly yields con- 
siderably at first by its elasticity without breaking, and that 
if the weight be lifted the surface will recover its flatness. 
The point should be taken when the meniscus of depression 
suddenly rises, showing that the sinker has penetrated. 

I find it more convenient to employ, instead of a pointed 
rod, a round-ended thin glass tube which can be weighted 
with mercury or shot, and to graduate the containing vessel 
in millimetres. It will be found that as the weight is 


gradually increased, the tube will penetrate the jelly a certain 
distance and then stop ; with careful further increase it will 
advance further and again stop, and so on till a weight is 
reached which carries it to the required distance (100 mm. 
in Kissling's method). These positions of equilibrium are 
due to (1) the resistance by friction and adhesion becoming 
greater as the tube sinks deeper, owing to the larger surfaces 
in contact ; (2) to the forcing up of a column of broken jelly 
in the cylinder. The latter effect is greater in a narrow 
cylinder. It was attempted to remove the former by oiling 
or greasing the rod, but the result could not be made uniform. 
The best method of working was found to be as follows : — 

Having first observed the ** bearing weight," i.e., the 
weight required (tube + mercury or shot) to break the sur- 
face, gradually increase it till a weight is found which will 
force the sinker uniformly through the distance. Place this 
weight at once on another cylinder of jelly, and record the 
time taken to traverse the distance. 

Experiment 1. — Cylinders of different diameters were 
filled with a 5 per cent, solution of a high grade gelatine, 
and cooled for two hours in ice and water. The sinker was 
11 mm. diameter, the cylinders 22, 24, and 26 mm., hence 
the annulus of jelly was 11, 13 and 15 mm. The results 
were : — 

A 416 grammes sank 40 mm. in 1 minute : annulus 11 mm. 
B it >» 80 „ „ ,, 18 „ 

C }) „ 26 t, „ }i 16 „ 

Mean 100 millimetres in 197 seconds. 

It will be noticed that 

40 X 11 = 440 
30 X 13 = 390 
26 X 16 = 376 

therefore the effect of the width of the vessel diminishes as 
the diameter increases, showing that a wide vessel should be 


used where possible. Kissling adopts 80 mm. diameter and 
120 mm. height. 

EayperiTnent 2. — Diameter of sinker. A tube 17 mm. 
wide, mercury- weighted to 416 grammes in a cylinder of 
40 mm. diameter, sank 10 mm. in one minute and then 
became stationary. 

A tube 26 mm. diameter loaded to 815 grammes in a 
cylinder 80 mm. diameter would not sink into the gelatine 
at all. 

Ten mm. was found to be a convenient diameter. 

Experiment 3. — A 2J per cent, jelly from a second-class 
gelatine cooled four hours at 20° C. The weight of sinker 
required was found to be 92 grammes, and the times for 
sinking 100 mm. were 105, 81 and 99 seconds in three 
different cylinders of 26 mm. diameter. Mean 95 seconds. 

(The 17 mm. sinker of the same weight went down 45 
mm. in 170 seconds and then stopped.) 

The ** bearing weight " required to penetrate the surface 
of the jelly was 

Diameter of Sinker . 





Sectional area of Sinker . 




sq. millira. 

Bearing Weight . 





The weights required are not proportional to the sectional 
area. For bearing weight it is advantageous to use a wider 
sinker than for consistency. The latter is, however, a 
preferable determination as regards agreement of duplicates, 
and admits of fair accuracy when carried out under standard 
conditions. The values of the two gelatines used above 
would have been recorded as 197 and 95, which is consonant 
with their other characters. 

6. Weidenbuschy 1859 : Breaking Strain of a Rod made 
of Olue and Plaster of Paris, — Sticks of plaster cast of 
uniform size, saturated with glue and dried thoroughly are 
placed one by one in a metaUic ring having notches to 


receive them : a lever bears on the centre of the stick as the 
resistance, and a mercury cup on the long arm forms the 
weight. Mercury is poured into the cup till the stick com- 
mences to break. The weight required is recorded, and 
compared with a standard glue gives the ** Weidenbusch 
figure ". For special purposes where glue is required as a 
body this would seem to be a practical method, but it is 
somewhat uncertain. 

7. The Spandau Teat, — Karmarsch proposed, and the 
Artillerie Werkstatte, Spandau, has adopted, the severing 
strain of two blocks glued together. They are usually made 
of well-seasoned mahogany, in America of maple, and the 
parting is caused by a lever, clamps, and weights, or better 
by a system of levers in a cement-testing machine. For the 
important objects of joinery and cabinetmaking this would 
seem to be an eminently natural process, all the rest being 
more or less arbitrary and artificial, except with reference 
to covering power and economy. But by a great number of 
experiments it was found that the error in duplicates was 
often very large, as has been remarked by numerous ob- 
servers. Hence, I undertook an investigation of the causes 
of inaccuracy. These were found to consist of : (a) Incon- 
stancy of material and surface in the blocks to be severed. 
The following figures are given for holding power in kilo- 
granames per square centimetre : — 

Cat Across the Parallel to the 

Wood. Grain. Grain. 

Beech 166 79 

Hornbeam 126*6 79 

Maple 88 63 

Oak 128 56 

Fir 110 24 

The hold on wood cut parallel to the grain being the 
same whether the fibres are laid parallel or crosswise. 

(6) Time of heating the glue and temperature of glueing. 


thickness of glue interposed, pressure used (with occasional 
obliquity in applying the pressure, therefore inequality in 
the layer), presence of lumps. 

(c) Moisture in the atmosphere and temperature while 
setting, and duration of rest before trial. 

(d) Temperature, etc., while testing. 

(e) Possible jerking in applying the weight. 

Kissling substituted for wood two vertical solid cylinders 
of nickel-plated iron, with one end smooth and the other 
with a hook for applying the strain. But there is a great 
practical objection to using an absolutely non-porous material 
for the surfaces to be glued. On examining the fracture 
it will be found that it usually occurs between the glue and 
the metal, therefore the measurement is simply that of the 
adhesion of the glue to the metal ; whereas, in practice, if a 
joint properly made be examined after rupture, it will be 
found that the parting takes place through the substance of 
the glue layer itself, about half of which remains attached to 
each face of the >yood, so that the test is a practical measure 
of the cohesion of the glue. Eoughehing, lining or grooving 
of the metallic surface does not remove this objection. 

On the other hand, in severing glued wood, it frequently 
happens that the fracture breaks into the substance of the 
wood itself, the latter having a lower tenacity than the glue : 
here the test is obviously valueless. 

Bauschinger devised a sliding or lateral test, in which 
two pieces of red beech wood, a decimetre square, were 
glued together with their fibres parallel and their surfaces 
over-lapping one centimetre. The pieces were fixed in a 
Werder's testing-machine, and forced in a direction parallel 
to their surfaces, till the glue yielded, and the pieces slid over 
one another. This test is only advisable for special uses, as 
friction interferes, and the wood itself frequently breaks; 
moreover, in joinery, a shde is usually stopped by a mortise. 


Horn determined the resistance to a tearing force of the 
substance itself, as: Common glu6, « 9*6 kilos per square 
centimetre; Cologne, 10'6; gelatine, 31*5: this would greatly 
depend on the hygroscopic state. 

For special objects, Setterberg {Schwed. technisk Tides- 
krift, 1898, xxviii., 52) soaks strips of paper in the glue 
solution, removes the excess with filter paper, allows to dry, 
and then mounts the strips in a paper-testing machine. 

By a large number of experiments I was led to adopt 
the following process and precautions : — 

For the testing blocks a hard yet moderately porous bis- 
cuit stoneware was found to be a much better material than 
wood. They were made to our design by Messrs. Doulton, 
and were of the shape of a truncated wedge fitting the claws 
of the machine — a glue-tester by Bailey of Salford, with, a 
system of levers, and a weight tub loaded with shot which 
was afterwards weighed. The opposing planes showed a 
fine uniform grain ; they were accurately ground to the 
square inch. After using, they were boiled clean, and dried 
for further use. 

One part of broken glue soaked in 2 parts of cold 
water for six hours, then raised to 70° C, agitated in a 
covered vessel till dissolved, and rapidly strained through 
fine muslin. The plane surfaces were soaked in this for 
thirty minutes at 70° in a covered vessel, joined care- 
fully with light pressure, the pairs mounted in a frame 
between uprights, each pair being cautiously loaded with a 
weight of 5 lb., and the whole kept for five days in a cool 
and dry room. At the end of that time they were mounted 
in the machine and tested, the utmost care being taken to 
avoid jerking, to put on the weight gradually, and to keep 
the lever horizontal. An improvement on shot would be 
a hopper with dry sand or mercury flowing through a regu- 
lated orifice with an automatic cut-off at the moment of 


fracture, which is generally sudden. Supports must be 
placed just underneath the weight tub. 

Example. — A brown Scotch glue of excellent character 
gave, with four different pairs of stone blocks, a breaking 
strain of 675 lbs., 636 lbs., 712 lbs., and 645 lbs. per sq. in. 
The average of all the mahogany blocks (ten experiments) 
was 331 lbs., but the single experiments varied from 70 lbs. 
to 668 lbs., showing scarcely any regularity or certainty. 
The stone, on account of its lower porosity, worked best 
with 50 per cent, glue, the mahogany with 20 per cent. ; a 
thin film of glue remained attached to both stone surfaces 
after rupture. The greatest advantage that stone blocks 
possess is their rigidity , the largest source of error in wood 
blocks being due to the elasticity and compressibility of the 
wood, and consequent yielding and shpping during the test, 
which often brings about the fracture prematurely, and 
occasions abnormally low results. 

A rough method of glue testing without apparatus is 
to cut transversely with a fine saw a stick of wood 5 
centimetres (1 inch) square, to glue the cut surfaces 
together and mount them on end with a moderate weight 
on top. After three days, the bar is laid across the supports, 
and weighted gradually in the middle till fracture ensues. 
Good glue should sustain a weight of at least 75 kilogrammes. 

8. Viscosity or ** Body " Test. — An American manufac- 
turer sends me the annexed description of this process as 
practised by their firm : — 

*' One oz. of glue is soaked thoroughly in 10 ozs. of water, 
melted in the water bath, poured into a * testing tube ' kept 
at a standard temperature, and the time observed that is 
taken in running through. * Water requires 37 seconds while 
the very weakest glue takes 40 to 43 seconds.' The ratio is 
the * body test ' — * indicating the consistency of the glue and 
the amount of surface it will cover, and, therefore, its cheap- 



nes& in use and good working and water-taking qualities '. 
We attach more importance to this test than to the con- 
sistency of the jelly." The above determination would show 
a ratio in 10 per cent, solution, taking water as a standard^ 
of ^ for the very weakest glue, or in decimals 1*16. 

But in the method of noting the time taken by 50 cubic 
centimetres of a 10 per cent, solution to run out from a 

Fig. 18. Engler's Viscosimeter. 

burette fitted with a fine jet, as compared with the time 
taken by water under the same conditions, the numbers 
obtained are not strictly proportional to the true viscosity, 
and vary so much with slight differences of temperature that 
very serious errors may arise if the operator, working on 
different days, does not use for comparison all the standard 
glues he had previously employed. 



For this reason Julius Fels employs Engler's viscosi- 
meter (improved form, Joum, Soc. Chem, Industry, 1890, 
p. 654) with a 15 per cent, solution, and subjoins this table : — 

I'lme of efflux 

Percent, of 

of 600 cc. at 30° 


Description of Samples. 

moisture in 

C. in seconds, 
water taking 



90 seconds. 

Light yellow transparent thick 

plates .... 





Brown transpaj-ent glue . 





Sherry-coloured transparent glue 





Light yellow plates, brittle 





Muddy (triiber) glue . 




He remarks that ** the viscosities found agree with the 
respective behaviours under water. No. 2 became entirely 
slimy in a few hours and fused into one lump, whereas No. 5 
kept its shape, and on scraping with the finger showed 
scarcely any gelatinisation. Nos. 3 and 5 in twelve hours 
gave a thick jelly, while No. 2 in twenty-four hours yielded 
only a poor and thin result." 

The ordinary preference for a dark glue is justified by the 
above figures. No. 2 seems to have been over-dried. Lighter 
glues may have been bleached at the sacrifice of tenacity. 

In determinations of viscosity I use and prefer an apparatus 
modified from Slotte's form, which I have described and 
figured in the Journal of the Society of Chemical Industry, 
1891, p. 615, as being simpler, cheaper, more manageable, 
and giving the datum in terms of absolute measurement. 
The numbers given by different observers have been obtained 
at different temperatures, and with different strengths, hence 
are not strictly comparable. Thus, Fels worked at 30° C. 
with a 15 per cent, solution, the Americans with 10 per cent. 
'* at a standard temperature '\ Therefore conditions should 
also be stated. I have been accustomed to use a 1 per cent. 



Fia. 14. Bideal-Slotte Viscosimeter. 


solution at 18° C, operating in the way mentioned in the 
above paper. 

The three bulbs are kept by the water jacket at the 
temperature required. The glue solution, also at the right 
temperature, is drawn up through the capillary tube from 
the lower bottle till it just fills the upper smaU bulb. Then 
the suction tube is removed, the upper glass tube immediately 
closed with the finger, and the solution cautiously run down 
to the upper mark of the large bulb. At this point the time 
is noted, and the liquid at once allowed to run till it reaches 
the lower mark, when the exact time is again noted. The 
difference is the number of seconds required to empty the 
large bulb under atmospheric pressure. For very viscid glues 
the time may be hastened by applying suction below as shown 
in the figure. In this case the gauge of the pump must be 
read and a correction applied, except for immediate compari- 
sons. The complete formulae and corrections are given in 
the paper. It is easy from them to make a table for at once 
translating the data observed into absolute viscosities, or 
more conveniently, into the relations to water taken as 100. 

It is essential to pass the solution first through a capillary 
tube by pressure to remove lumps, which are more frequent 
in ** muddy " glues — some of the high figures found for this 
class (e.g.y the 2*21 in Fels's table) may be due to this feature. 
Straining through mushn is not sufficient, but should be used 
as a preliminary. My figures for different varieties (1 per 
cent, at 18° C.) have ranged from 1*19 to 1*6. 



EiRsling has recently given the following table of tests for 
different glues {Chem. Zeitung, xxii. [19] , 172) : — 



5 S.-t 






Smell of 
soaked glue. 


(rod method) 

over 1000 



Skin glue . 






»» • • • 






i» • • • 


very good 





Bone glue in powder 







>> »» • 






^» ^» 






»» »» 







Mixed glue 







Bone glue in cakes . 






n »» 


very bad 



>» 11 


rather bad 





>> n 






This author remarks that ** the smell is especially impor- 
tant, as indicative of the keeping properties. (Compare No. 3 
and No. 12 above.) In adhesive power cheap bone glues give 
almost as good results as the high-priced skin glues. If a 
. glue buyer requires good smell, gelatinising property, and 
freedom from acidity, a best skin-glue must be selected. If 
only the adhesive properties are to be considered, a cheap 
bone glue will answer the purpose.'' 

** Fels's method — that is, the determination of the vis- 
cosity — can give very useful indications, but there are certain 
reservations which must be made. Solutions of glue undergo 
— after a more or less prolonged time of standing — modifica- 
tions in fluidity. In this way a freshly-prepared solution had 
a viscosity of 90, but after standing for twenty-four hours it 
had assumed a gelatinous consistency. Another solution 
underwent the following modifications : Its degree of viscosity 
was 3*7 after two hours, 4*8 after twenty-four hours, and 5*4 
after forty-eight hours." 

9. Foa/m Test, — A 10 per cent, solution is beaten or stirred 
vigorously for three or four seconds with a small glass rod. 


the height of the foam measured in inches, and the rate of 
its disappearance noted. ** Some glues show ^ in. foam, 
some 1^ in., 1^ in., and some none at all. It does not neces- 
sarily follow that there is anything wrong with the glue, 
but a great many customers object to foam." For some 
purposes frothing is a great disadvantage. 

Valuation of Raw Materials. 

The value of glue-making materials, more especially of 
tanners' " spetches " (refuse and parings), depends not only 
on the absolute amount of glue-forming substance, but also 
on its relation to the non-gelatine. Gantter boils 100 
grammes of the sample vsrith 1 litre of water and a few drops 
of soda till completely extracted, makes up to 2 litres, and 
allows to stand covered in a warm place for ten hours. The 
insoluble matters settle, while the fat rises to the surface. 
The residue and ash are determined in 20 cc. ( = 1 gramme 
of substance). Another 20 cc. are diluted, neutralised with 
acetic acid, precipitated by tannin, filtered, and any excess 
of tannin withdrawn by hide-powder ; the liquid evaporated, 
dried, and its ash deducted, gives the organic non-gluemaking 

It may be said in conclusion that a careful inspection of 
the results of the scientific examinations of a glue or of raw 
materials gives frequently a better criterion of its value for a 
definite purpose than the price, as the latter is often deter- 
mined by hand tests and by artificial considerations. 

The following is an example of the examination of two 
typical glues, with the conclusions derived : — 

(1) Chemical Examination : — 

Best Glue, 1. Common Glue. 2. 

Moisture . . 14*32 per cent. 13'41 per cent. 

Ash . 6-04 „ 1-77 „ 


The ash in each case contained lime, phosphoric acid, 
and chloride indicating the presence of bone glue. 
(2) Mechanical Examination : — 

Best Glue, 1. Common Glue, 2. 

Water absorbed after 

soaking in the cold 

for forty-eight hours. 600 per cent. 170 per cent. 

Condition after soaking. Firm and gelatinous. Semi-fluid. 

Odour of solution after 
heating. Slight, inoffensive. Slight, inoflensive. 

Viscosity of 10 percent, 
solution at 60° F. 
compared with that 
of normal rape oil 
at the same tempera- 
ture taken as 100. Solution too gelatinous 

to flow. 15. 

Tensile strength be- 
tween test blocks of 
mahogany, lb. per 
square inch. 511. 227. 

In spite of the slightly higher percentage of water, and 
the considerably greater amount of ash which it contains, 
sample 1 is a quality so much superior to that of sample 2 
that it would be more economical to buy even at the rate of 
£38 per ton as against £24 per ton for the other. 

From the tensile strength figures, it follows that the best 
glue is better in the ratio of 13 to 9 per unit price, i.e., for a 
given sum of money the best glue is 60 per cent, better. 

As in many other trades, a great deal of reticence is 
practised by glue and gelatine manufacturers with regard to 
the details of their special processes, although in all cases 
the processes must be broadly the same. But in this work 
special care and attention to minutiae is required at every 
stage, and when by long experience a particular recipe is 
found to be successful, there is every excuse derived from 
competition for keeping it carefully concealed. And yet the 
policy of ** trade secrets " has of late years undergone a 


considerable change. It has been recognised that the open 
publication of scientific researches has been of incalculable 
advantage to manufacture, while it has been also slowly 
found that many jealously guarded recipes of the *'rule of 
thumb *' order were either unnecessary or absolutely detri- 
mental. Communication and discussion of the various 
processes, such as manufacturers and chemists have estab- 
lished in the ** Society of Chemical Industry " has led to 
great improvement in methods and economy of production, 
as it has become increasingly evident that scientific precision 
was indispensable for attaining the best and most uniform 
results. It is difficult to understand why practical men, 
especially in the British Islands, so rarely appreciate the 
advantage of working under exact conditions of temperature, 
time and quantities. While in the best manufactories 
attention to these .points is very carefully practised, the 
private workman is usually content with the crudest '* rule 
of thumb *' methods, with the result of occasional most 
irritating failures, which are either wrongly put down to the 
glue or attributed to the weather or other cause. For 
makers, buyers and users alike, an intelligent knowledge of 
the subject is indispensable. 



As will be gathered from our previous remarks, glue, gelatine 
and size are looked upon as subsidiary or bye-products to the 
leather industry, and receive only casual reference in trade 
journals. For this reason we find no reference specially in 
the Board of Trade Journal or in Consular reports to glue 
or gelatine as separately tabulated articles of commerce. The 
importance of these articles to the joinery, cabinetmaking, 
paper, printing, and other industries, would seem to indicate 
a different treatment. The fault is largely due to the extra- 
ordinary secrecy observed in the glue trade, whereby, while 
other manufactures have been worked up by scientific 
investigations to the maximum of efficiency, the making of 
glue has remained traditional, with local maxims and local 
errors. For instance, a leading manufacturer writes : — 

** The manufacture of glue being in great part a secret 
one, each manufacturer having different methods of preparing 
his articles, records of the manufacture are simply matters of 
knowledge handed down from father to son amongst the head 
men of the trade ". 

Another states that : — 

** Every one must make their own experience, which in 

most cases in our trade has been very dearly bought ". 

Consequently, whereas in other greater trades the publicity 



has led to scientific researches which have proved of advan- 
tage to -tb^ production generally, in the glue trade the rather 
antiquated policy of exclusiveness, secrecy, and dread of 
competitors has succeeded in keeping the rationale of the 
processes to a certain extent unknown, the information 
scanty, and improvement, particularly in the English trade, 
decidedly backward. " Trade secrets," as a rule, are of very 
little value, and often, when leading to a stereotyped mode of 
procedure, are a hindrance to advance. As in other chemical 
trades, the object is to produce the greatest amount of a 
definite substance of the highest quality consistent with 
price. There must be, at most, one or two best ways of 
doing this : any deviation from these must result in loss, 
and can only be avoided by a uniform method being, as 
much as possible, adopted, the precautions carefully worked 
out and tested with exactness at every stage. This could be 
done by collaboration, as in other industries, when the glue 
and gelatine trade would be saved from the minor position, 
and the uncertainties and fluctuations it has shown in the 

There is a great difficulty in obtaining statistics of the 
glue trade. As we have already mentioned, glue, gelatine, 
size, isinglass and *' stock," do not appear in the British 
Board of Trade Eetums, and are probably massed as imports 
in ** raw materials for non-textile industries," and as exports 
in ** miscellaneous " or ** other articles ". Scanty and scat- 
tered information is found in the official reports of foreign 
countries, most of it discouraging. Thus in a German report 
of 1893 it is said :— 

** The bone meal and glue industry sustained a loss owing 
to Russian competition and the increased sale of superphos- 
phates" from mineral sources. ** Many bone-ash works 
betook themselves to the manufacture of glue, thus further 
depressing the position of that industry." 


And again in a report of the same country for 1894 : — 
** Gltue lost the Austrian and Dutch markets, but found 
compensation in England, the States, and English North 
America ; the prices, however, receded from 100 to 70 marks 
per 100 kilos (£51 to £35 10s. per ton). The imports are 
valued at 25 marks less (1893 ; 15 marks), and reached 
a maximum in 1890, 1734 tons in 1892, and 1958 tons in 
1893," In the same report it is stated: ** The exports of 
gelatine lost ground in Holland, Belgium, and Eussia, but 
this was more than compensated for by the increased demand 
in England and the States. There is a simultaneous decrease 
o£ imports,'* with values receding for exports from £157 to 
£112, and for imports from £147 to £102 per ton. 
A prominent member of the trade writes : — 
** In England there are certainly no statistics available 
as to the production of glue, which is an uncertainty, and 
of regular irregularity.** He probably refers to the long- 
established habit of suspension of work while climatic cir- 
cumstances are unfavourable. ** Works close and open, work 
in fits and starts. There are bone-glue works— these make, 
at least some of them, mixed glues also, i.e., hide and bone 
and horn piths — and purely hide-glue makers. A number of 
tanners work up into glue their own production of hide pieces. 
Paper makers work up hide pieces into size for their own 
consumption. All the world over a similar state exists." 

There has always been great competition between the 
manufacturers of hide and of bone glue. While the former 
has always been ranked higher in the carpentry and cabinet 
trade, and has commanded a better price, it has been recognised 
that, owing to the chemical improvements in the treatment 
of bones, the glue from them has been nearly equal in 
tenacity and often better in colour, so as to be more suitable 
for many purposes, at the same time being sold at a lower 
price ; hide glue averaging about £46 per ton, and bone £35 


to £21 or lower. Many other products were also obtained 
from bones, such as bone meal, superphosphate, fat, animal 
charcoal, ammonia, bone oil and phosphorus, and were ex- 
pected to make the process highly remunerative. For a time 
there was a considerable demand for bone or ** patent " glue, 
especially in France. But glue still remained **a drug in 
the market *' till the latter part of 1896, when an impulse was 
given to the hide glue trade by the failure of a number of 
bone glue works in Germany, some of them with large pro- 
ductions. So-called ** small cake patent glue," benzene 
bleached, was thrown on the English market in quantities 
for cash at prices down to £14 per ton delivered. From 
that time an improvement dates. The same class of glue 
rapidly rose to £28 per ton, which is now about the price. 
At present there is a tendency towards improvement, and 
larger purchases may be expected both from America and 
elsewhere. But in spite of the improvements in quality at- 
tained by better machinery and closer attention to processes, 
it is complained that English buyers still continue to take 
glue by superficial characters, such as shape, fracture or gloss, 
instead of by real quality, and that in consequence imitations 
are easily passed off. On the Continent more attention is 
generally paid. 

The trades both in America and Holland are syndicated, 
so that the production is to some extent controlled. In 
America, the trust, by avoiding cutting prices and the cost 
of commercial travellers, is endeavouring to maintain former 
prices, and so make the business more remunerative. 



From the Board of Trade Beturns the imports of hides 
into the British Isles for the last ten years have been : — 


Raw Hides, Dry. 

Baw Hides, Wet. 



Value. £ 



Value. & 













The imports of hides into Liverpool for the week ending 
July 7th, 1899, are given as : 795 salted Eiver Plate, 11,250 
salted Italian, 2,000 salted Brazil, 232 salted Oporto, 1,700 
dry African, 400 dry West Coast, 4,728 dry E. Indian 
Buffalo, 66 bales dry Egyptian. Number of hides in markets 
at time of auctions, 2,534. Corresponding week in previous 
year, 2,952. 



Acetic gelatine, 95 

Acid process for bone glue, 49 

Acids, fixed, 109 

— volatile, 109 
Action of ozone on glue, 58 
Adulteration of chocolate with 

gelatine, 116 

— of gelatine and glue, 116 

— of meat extracts with gela- 

tine, 117 
Agar-agar, 103 
Albumen, clarification with, 56 

— glue, 97 
Algin, 103 

Alum, clarification with, 55 
Animal size for paper-making, 71 
Antiseptics, 39 
Apoglutin, 80 
Artificial leathers, 77 


Bacteria, action of, in salting and 

liming, 38 
Bacteriology, use of gelatine in, 

Bauschinger's test, 125 
Bearing weight of jelly, 119 
Bisler-Beumat tannin titration, 

Blasting gelatine, 89 
Blue size, 85 
Bone ash, 3 

Bone glue, acid process, 49 
Bookbinders' size, 73 
Boric acid as a preservative, 40 
Brazilian isinglass, 101 
Buckshorn as a source of glue, 24 
Buller's glue compound, 88 
Bullock's feet as a source of glue, 21 

Buttner's apparatus for glue ex- 
traction, 46 


Capsules, gelatine, 105 
Carbon photographic process, 88 
Casein glue, 97 
Cayenne isinglass, 101 
Chemical methods of glue test- 
ing, 107 

— reactions of gelatine and 

glue, 79 
Chinese moss, 103 
Chitin, 8 
Chitinoids, 8 
Chloral hydrate, use of, in liquid 

glues, 96 
Chondrigen, 7 
Chondrin, 8 
Chrome glue, 88 
Clarifying, 55 
Cleansing vessels, 57 
Collagen, fermentation of, 2 
Collagens, 2 
Coloured-paper manufacture, use 

of gelatine in, 90 
Compositions for printing rollers. 

— — hectograph beds, 77- 
Conchiolin, 8 
Consistency of jelly, 119 
Continuous process for forming 

and drying glue sheets, 60 
Cooking, 40 
Cooling, 56 
Coriin, 23 
Corium, 23 
Corrosive sublimate disinfection, 

Covering power of glue, 65 
CrosskiU bone mill, 49 




Crushed glue, 62 
Culinary gelatine, 98 


Detannated wines, 104 
Diamond cement, 95 
Dippers oil, 64 
Distemper, 86 
Drying of glue cake, 60 


Elastic glue, 97 
Elasticity of glue, 65 
Elastin, 3 
Epidermis, 23 
Exports, 138 
Extraction, 40 

— by Bertram's process, 43 

— by closed tank process, 43 

— by English process, 44 

— by French process, 43 

— by open process, 41 


Fat in bone, 3, 64 

— determination of, 112 

— extraction, 62 

Fels's method of determining vis- 
cosity, 129 
Fermentation of collagen, 2 
Fibroids, 8 
Fibroin, 8 
Filter presses, 30 
Finings, 102 

Fixed acidity in glue, 109 
Flaked gelatine, 89 
Foam test, 132 
Foaming, causes of, 70 
Formaldehyde as a preservative, 

— detection of, in glue or gela- 

tine, 93 
Formalin applied to photographic 
films, 92 

— in sheet gelatines, 91 
Forming the glue sheets, 57 
Formo-gelatine, 90, 92 

— artificial silk from, 93 

— preparation of, 92 
Fractionated boiling, 41 


Grelatine, 1, 5 

— and glue, distinction be- 

tween, 10 

— and glue, Huet's process for 

manufacturing, 45 

— capsules, 105 

— chemical reactions of, 79 

— description of, in British 

PharmcLCopoda, 104 

— determination of, in glue, 


— effect of heat upon, 13 

— flaked, 89 

— for coloured lights, 105 

— in photography, 82 

— in printing roller composi- 

tions, 74 

— in substitutes for other 

materials, 75 

— physical properties of, 81 

— preparation of, from bones, 4 

— preparation of pure, 6 

— -producing substances, 7 

— scented, 90 

— sulphur in, 5 

:— use of, in paper-making, 70 

— use of, in straw hat manu- 

facture, 71 
Gelatones, 4 
Gelatoses, 4, 81 
Gelose, 103 
Glue, 1, 10 

— Cologne, 19 

— effect of heat upon, 13 

— fish, 20 

— French, 19 

— from leather waste, 37 

— gilders', 19 

— grades of, 15 

— in bookbinding, 71 

— in coloured-paper manufac- 

ture, 90 

— in match manufacture, 75 

— parchment, 20 

— Paris, 20 

— patent, 19 

— physical tests of, 117 

— preparation of, from fish, 

whale, etc., 4^ 

— Russian, 18, 19 

— size, 20 

Glues, classification of, 14 



Gluten glue, 97 

Glutei, 92 

Glycerine in glue or gelatine, 116 

— jelly, 106 
Groth's process for preparation of 

glue, 46 

Hectograph beds, composition for, 

Hewitt's process for forming and 

drying glue sheets, 61 
Hides, importation of, into British 

Isles, 140 
Hoeveler's glue stock washer, 36 
" Hollander" machine, 38 
Horn cuttings as a sourcie of size, 

Huet's process for manufacturing 

gelatine and glue, 45 


Importation of hides into the 

British Isles, 140 
Injurious effect of thunderstorms 

on glue, 58 
Irish moss, 103 
Isinglass, 7, 101 
Isinglassine, 103 

Japanese isinglass, 43 
Jellies, 7, 10, 98, 99 


Keratin, 8 
Keratoids, 8 

Kissling consistency test, 120 
Kjeldahl process, 114 
" Knaffl's glue," 95 
Kranseder & Leutsch's drying 
trays, 61 


LamellsB, 105 
Leather, artificial, 77 
Leuner's apparatus for fat extrac- 
tion, 62 

Lime, solubility of, at different 

temperatures, 30. 
Lime solution, making up of, 29 
Lime, test for strength of, 28 
Liming, 25 

— object of, 26 
Lipowitz*s test, 119 
Liquefaciens, B., 81 
Liquid gelatine, 81 
Liquid glue, 94, 97 


Machalski^s apparatus for glue 

extraction, 47 
Mineral glue, 96 
Mineral salts in specially prepared 

glues, 86 
Mirrors, protection of, 89 
Mucin, 9 


Nawrocki's apparatus for glue ex- 
traction, 46 
Nitrogen, determination of, 114 
Nutritive value of gelatine, 99 


Opaque gelatine, 87 
Optical properties of gelatine, 82 
Ossein, 2 

Oxyacids, fluid, in glue, 113 
— solid, in glue, 113 


Paper-making, use of gelatine in, 

Patent isinglass, 99 

Phenol as a preservative, 40 

Photographv, use of gelatine in, 

Physical methods of glue testing, 

Physical properties of gelatine sol- 
utions, 81 

Porous glue, preparation of, 62 

Powdered glue, 62 

Powders, addition of, to glue and 
gelatine, 87 

Preparation of glue for use, 67 

Prepared casein for sizing, 71 



Prices, 134^ 138 

Printing-roller compositions, 74 
Proteids, 2 
Proteoids, 2 

Tuncture fluids for pneumatic 
tyres, 98 


Raw materials, 21 

— — valuation of, 133 
Reaction of glue, 109 
Recovery of sulphurous acid, 65 
Richards' steam glue oven, 69 
Rideal-Slotte method of deter- 
mining viscosity, 131 


Saponification process, 113 

Scented glue and gelatine, 90 

Secrecy of the trade, 136 

Sedimentation, disadvantage of, 

Seltsam's process of fat extrac- 
tion, 62 

Sericin, 7 

Setterberg's test, 126 

Size, 15 

— recipes for, for bookbinders, 

Skin glue, sources of, 21 

Skins, composition of, 23 

" Small cake patent glue," 139 

Smell of glue, 117 

Soda, use of, in glue-making, 36 

Spandau test, 124 

" Spetches," 133 

Spoilt and waste bones, use of, 64 

Spongin, 8 

Statistics, 127 

Stel ling's process, 112 

" Stock," various kinds of, 21 

Subtilis, effect of B., on gelatine, 

Sugar in glues, 87 

Sulphide of sodium in glue-mak- 
ing, 37 

Sulphur in gelatine, 5 

Sulphurous acid disinfection, 40 

— — process for bone glue, 

Swenson's patent multiple eva- 
porator, 49 


Tannery waste for glue, 21 

Tanno-gelatine, 88 

Telescopes, treatment of, with 

bichromated gelatine, 89 
Trade statistics, 137 
Tub-size manufacture, 84 


Utilisation of leather waste and 
cuttings for glue, 76 
— — — — various pur- 
poses, 76 


Vacuum pans, 48 

Valuation of raw materials, 133 

Vanduara silk, 93 

Vegetable glue, 25 

Veneers, 17, 69 

Viscosity, 127 

Volatile acids in glue, 109 


Washer, Hoeveler's, 36 
Washing limed stock, 35 

— salted stock, 36 
Water supply, 33 

— artificial treatment of, 35 

— filtration of, 35 

— quantity of, 33 

— quality of, 33 
Water-absorption power of glue, 

"Waterproof glue, 89, 97 
Weidenbusch breaking - strain 

figure, 124 
Wheat gluten, detection of, in glue» 


Yaryan multiple evaporator, 49 


Zinc sulphate, use of, as an anti- 
septic, 57